Changes

testenv: support running SUT in QEMU

Add two new arguments -C|--custom-kernel and -D|--debian-kernel. If any
of these is set, pass an environment variable TESTENV_QEMU_KERNEL with
the path to the kernel when running commands from testenv.cfg.

These commands can then source the new qemu_functions.sh and use it to
build an initramfs with the SUT and depending libraries on the fly, and
start up QEMU to boot right to starting the SUT. All of that takes about
~1s on my system with kvm. Without kvm ~5s.

A follow-up patch will adjust the ggsn testenv configs to optionally run
osmo-ggsn in QEMU for testing kernel GTP-U.

These scripts are based on scripts/kernel-tests from docker-playground.

Related: osmo-ci Id64a1a778fa38eec20498c36b390332f75d7d3f5
Change-Id: Ic9cb7092fd029b7ba530fc755b5d4d73a9d86350