Changes

scripts/kernel-test: do shallow fetch

Instead of fetching the entire history, only fetch the most recent
commit.

Change-Id: I92d90c1451ff6abff2df3b1318b5194880e77f2e

kernel-test/kernel-build: update net-next branch

The branch of net-next is now called main. Update the default
KERNEL_BRANCH to main, as the default KERNEL_URL points to net-next.git.

Related: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/
Change-Id: Ic4b13c3cbc0933db9d520e6d737c45b6b08e8c48

jenkins-common: fix_perms: chown files too

Ensure the user that runs docker owns files in cache and data dirs
during fix_perms, if an old job created them as root.

Change-Id: I43af48bd87e27abad5ad52e734256d78651f67c3

scripts/kernel-test: put mknod commands in init.sh

Prepare to run the preparation without root rights.

Change-Id: I07648f058ca55515ffc7396598147a03dcfe7976

scripts/kernel-test: build initrd+kernel as user

Refactor code to build the initramfs and kernel as user, instead of
running related code as root. Put the smoke_test.log into /tmp instead
of the cache dir, so we don't write as root into the cache dir. We cat
it on error anyway, so we don't need to have it in the workspace.

Change-Id: I1670dd86f2670f42eb99e4527d483965d8222402