scripts/west/sign: Auto-detect rimage ELF files

This code had a growing "enumerate all the platforms" wart.  In point
of fact only intel_adsp is special here.  Other rimage platforms don't
have a bootloader and use zephyr.elf directly.

Don't hard code platform names (especially since they aren't the same
as board/soc names!).  Just autodetect which scheme we have at
runtime.

Signed-off-by: Andy Ross <andyross@google.com>