Changes

scripts/kernel/linux-shallow-clone: new script

Add a script that maintains a linux repository in one place on jenkins
nodes, so we need less git clones from git.kernel.org and less disk
space. All jobs that need a kernel tree can now clone the relevant
branch directly from the jenkins node.

Follow up patches will add a jenkins job that runs the script daily and
adjust the existing jobs to make use of this instead of doing their own
clones.

Currently this script produces a 396M bare git repository.

Related: OS#6938
Change-Id: Id3aadb46813047ecac3b80868192809b272dba0e