mxssb: Implement basic image verification

By using the -i option of MXSSB, one can now validate if the input
image is correct. MXSSB will also dump the contents of the image.
Signed-off-by: Marek Vasut <marex@denx.de>

README

@@ -12,8 +12,8 @@ Build:
 2) Compile the tool by running simple "make" command . The produced binary
    will be called "mxssb" and will reside in the current directory.
 
-Use:
-====
+Usage -- producing image:
+=========================
 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:
 
@@ -109,3 +109,15 @@ These semantics and rules will be outlined now.
    J -- JUMP instruction
    C -- CALL instruction
    M -- MODE instruction
+
+Usage -- verifying image:
+=========================
+
+The MXSSB can also verify and dump contents of an image. Use the following
+syntax to verify and dump contents of an image:
+
+ mxssb -i <input bootstream file>
+
+This will output all the information from the SB image header and all the
+instructions contained in the SB image. It will also check if the various
+checksums in the SB image are correct.