common/armflash: load_image returns success or failure

Change the load_image so that it returns success or failure of the
command (using CMD_RET_SUCCESS or CMD_RET_FAILURE).

This way, hush scripts can optionally load different files depending
upon the system configuration.

A simple example:

if afs load ${kernel_name} ${kernel_addr}; then echo loaded; else echo \
not loaded; fi

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>