Move addresses for kernel, ramdisk and fdt blob to own variables. Add dtb
blob loading to all existing boot scripts, dtb filenames were taken from
vanilla kernel. Introduce new boot script bootcmd_legacy, which only loads
a kernel and a ramdisk. Make this the default boot script. This should also
restore the behaviour of the original bootloader.

Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Michael Walle <michael@walle.cc>