cmake: fix git describe command line

The "git describe" call for setting the boot banner is broken. The
--work-tree option sets the checked out work tree, not the directory
containing .git, which is where git describe needs to look for
information on tags.

Use -C instead so it's as if Git were run from the zephyr base
directory instead.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>