cli: Connect plan.config.gitsha to CONFIG.kernel_git_sha

The database schema has a field in the CONFIG table for the git sha
corresponding to the kernel source that was built. It is not possible to
get this info from a running kernel in any reliable way though, so to
date, the field has remained empty.

But when we start running developer branches through the system, they
may be subject to rebasing so we need a way to tell one version from
another when the kernel name may not be unique. So let's allow the user
to (optionally) provide the git sha as a config field in the plan. And,
if present, connect that up to the CONFIG.kernel_git_sha field. So now
configs will be treated as separate items if their git sha differs.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>