mxssb: Add README

Documentation is a must, add proper README file for this tool.
Signed-off-by: Marek Vasut <marex@denx.de>

README

+Freescale i.MX233/i.MX28 SB image generator
+===========================================
+
+This tool allows user to produce SB BootStream encrypted with a zero key.
+Such a BootStream is then bootable on i.MX23/i.MX28.
+
+Build:
+======
+1) Install libssl-dev or similar OpenSSL development package. The tool
+   is using OpenSSL to encrypt the image with zero key.
+
+2) Compile the tool by running simple "make" command . The produced binary
+   will be called "mxssb" and will reside in the current directory.
+
+Use:
+====
+The MXSSB tool is targetted to be a simple replacement for the elftosb2 .
+To generate an image, simply write an image configuration file and run:
+
+ mxssb -c <path to configuration file> -o <output bootstream file>
+
+The output bootstream file is usually using the .sb file extension. For
+early boot debugging purposes, a verbose output from the BootROM can be
+enabled by adding the "-v" option to "mxssb" command line. Note that the
+example configuration files for producing bootable BootStream with the
+U-Boot bootloader can be found in the MXSSB source distribution. See the
+following files:
+
+ mxsimage.mx23.cfg -- This is an example configuration for i.MX23
+ mxsimage.mx28.cfg -- This is an example configuration for i.MX28
+
+Each configuration file uses very simple instruction semantics and a few
+additional rules have to be followed so that a useful image can be produced.
+These semantics and rules will be outlined now.
+
+- Each line of the configuration file contains exactly one instruction.
+- Every numeric value must be encoded in hexadecimal and in format 0xabcdef12 .
+- The configuration file is a concatenation of blocks called "sections".
+  - Each "section" is started by the "SECTION" instruction.
+  - The "SECTION" instruction has the following semantics:
+
+      SECTION u32_section_number [BOOTABLE]
+      - u32_section_number :: User-selected ID of the section
+      - BOOTABLE           :: Sets the section as bootable
+
+  - A bootable section is one from which the BootROM starts executing
+    subsequent instructions or code. Exactly one section must be selected
+    as bootable, usually the one containing the instructions and data to
+    load the bootloader.
+
+  - A "SECTION" must be immediatelly followed by a "TAG" instruction.
+  - The "TAG" instruction has the following semantics:
+
+      TAG [LAST]
+      - LAST               :: Flag denoting the last section in the file
+
+  - After a "TAG" unstruction, any of the following instructions may follow
+    in any order and any quantity:
+
+      NOOP
+      - This instruction does nothing
+
+      LOAD     u32_address string_filename
+      - Instructs the BootROM to load file pointed by "string_filename" onto
+        address "u32_address".
+
+      LOAD IVT u32_address u32_IVT_entry_point
+      - Crafts and loads IVT onto address "u32_address" with the entry point
+        of u32_IVT_entry_point.
+      - i.MX28-specific instruction!
+
+      FILL u32_address u32_pattern u32_length
+      - Starts to write memory from addres "u32_address" with a pattern
+        specified by "u32_pattern". Writes exactly "u32_length" bytes of the
+	pattern.
+
+      JUMP [HAB] u32_address [u32_r0_arg]
+      - Jumps onto memory address specified by "u32_address" by setting this
+        address in PT. The BootROM will pass the "u32_r0_arg" value in ARM
+	register "r0" to the executed code if this option is specified.
+	Otherwise, ARM register "r0" will default to value 0x00000000. The
+	optional "HAB" flag is i.MX28-specific flag turning on the HAB boot.
+
+      CALL [HAB] u32_address [u32_r0_arg]
+      - See JUMP instruction above, as the operation is exactly the same with
+        one difference. The CALL instruction does allow returning into the
+	BootROM from the executed code. U-Boot makes use of this in it's SPL
+	code.
+
+      MODE string_mode
+      - Restart the CPU and start booting from device specified by the
+	"string_mode" argument. The "string_mode" differs for each CPU
+	and can be:
+         i.MX23, string_mode = USB/I2C/SPI1_FLASH/SPI2_FLASH/NAND_BCH
+                               JTAG/SPI3_EEPROM/SD_SSP0/SD_SSP1
+         i.MX28, string_mode = USB/I2C/SPI2_FLASH/SPI3_FLASH/NAND_BCH
+                               JTAG/SPI2_EEPROM/SD_SSP0/SD_SSP1
+
+- If the verbose output from the BootROM is enabled by passing the "-v" option
+  to MXSSB, the BootROM will produce a letter on the Debug UART for each
+  instruction it started processing. Here is a mapping between the above
+  instructions and the BootROM verbose output:
+
+   H -- SB Image header loaded
+   T -- TAG instruction
+   N -- NOOP instruction
+   L -- LOAD instruction
+   F -- FILL instruction
+   J -- JUMP instruction
+   C -- CALL instruction
+   M -- MODE instruction