west: sign: add support for NXP i.MX

Add support for signing i.MX SOF with Zephyr images with rimage.

Note that, for i.MX, we don't need a bootloader nor the .elf.mod files
since there is no need to change the VMA/LMA of each uncached section
to the equivalent address in the cached area of memory.
For the above reasons, I've updated the arguments given to "west sign"
command.

Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>