Changes

scripts/kernel-test: add KERNEL_SKIP_REBUILD

Speed up local test cycles by not building the kernel again from source
if not needed.

Change-Id: Iaa401918d7c09ec92ec546071d8961e99a4cbb47

scripts/kernel-test: add KERNEL_SKIP_SMOKE_TEST

Speed up local test cycles by not running the smoke test every time.

Change-Id: I505b74e8c9b221655a97c5464f6fe4f21204a894

kernel_test_prepare: use $DOCKER_ARGS

Use $DOCKER_ARGS in the docker run call to kernel-test/prepare.sh, which
builds the initramfs. I use this to make a local build of osmo-ggsn +
depends available in the docker container in /usr/local with the
ttcn3.sh script in osmo-dev.git.

Change-Id: Ie2500fa1bd9d57e2531463ce93fdd9930ad829d3