stand: Add a helper 'universe' target

Add a shortcut for invokging ${SRCTOP}/tools/boot/universe.sh by
creating a 'universe' target in src/stand. This will make it easier to
test out all the different combinations of boot loaders that we build.

Sponsored by: Netflix

(cherry picked from commit 206203f5301b1bda1ceabcb7ecb7c0eee3e6b07a)