This switches to cloning the ipipe tree, rather than downloading
patches. It allows to test also unreleased ipipe revisions.

The test matrix is enhanced by
 - 4.19-arm
 - 4.14.78-arm64
 - 4.14.y-x86
 - 4.4.y-x86
 - 4.4.y-arm

The 4.1-arm build is dropped because that kernel is no longer supported
- and the build is broken.

Furthermore, the no longer supported i386 target is replaced with x86-64
for 4.4.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>