Add support for "make universe_kernels -DMAKE_GENERIC_KERNELS" to build
just the GENERIC kernels for each arch (including variations such as
GENERIC-NODEBUG, GENERIC64, etc).

This helps with quickly doing a test build for all[*] arches without
building dozens of variant kernels for the arches that have lots of
hardware/board/system variations.

• Not all arches have a generic kernel (but they probably should for

test-building purposes, even if it can't boot on any real hardware).