common/armflash: add command to check if image exists

Add a command to the ARM flash support to check if an image exists or
not.

If the image is found, it will return CMD_RET_SUCCESS, else
CMD_RET_FAILURE.  This allows hush scripts to conditionally load images.

A simple example:

if afs exists ${kernel_name}; then echo found; else echo \
not found; fi

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