Change UnifiedKernelImages to enum and accept signed/unsigned

Change UnifiedKernelImages to enum and accept signed/unsigned

With custom firmware we enroll our keys in db, so local UKIs can be
built and there's no need to fail the build. Many distributions
ship signed bootloaders, but they still don't ship UKIs.
Add an enum and a parser (to keep backward compat), and if set to
unsigned build locally instead of failing when the bootloader is
signed.