firmware/skiboot: always specify GIT_SHA

When we're building skiboot in buildroot, we'll be using a non-git
tarball. skiboot's makefiles expect to grab the git version by calling
`git`, but that's not going to work with a tarball.

This change explicitly sets GIT_SHA for skiboot builds.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>