kernel.mk: update LINUX_VERSION filename for cloned repo

kernel.mk: update LINUX_VERSION filename for cloned repo

In case there is an external git repo specified,
it could overwrite the kernel tarball that was
downloaded from kernel.org.

The only identifier for such a file is the
KERNEL_GIT_CLONE_URI & KERNEL_GIT_REF symbols,
so if we have to download it we'll use that
information [after some sanitization]
to create a different filename for the kernel tarball.

If KERNEL_GIT_REF symbol is empty, HEAD will be used
as mentioned in the description of KERNEL_GIT_REF.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>