linux: don't take HEAD as default for git repositories

Specifying a floating tag like HEAD for a repository version is bad practice,
as it results in non-reproducible builds. This patch removes the default
assignment of HEAD as version when a custom git repository is used for the
Linux kernel.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>