Make it possible to run ttcn3-ggsn-test against osmo-ggsn with the GTP
kernel module. This feature is enabled by environment variables
explained in ttcn3-ggsn-test/jenkins.sh, and can either use the
pre-built kernel from Debian, or build a kernel from source.
The kernel runs inside QEMU inside the usual osmo-ggsn-master docker
image.
Thanks to Pau for providing the kernel config and kernel-test scripts
that this patch is based on! I ended up modifying them quite a bit to
fit the docker use case, and to build the initrd (so we can use the
stock debian kernel).
Related: OS#3208
Change-Id: I54e752048eed2a170a1cf4e0fd410de255b9de5d
