kernel-flavor: Add linux-image-generic-lts-xenial

kernel-flavor: Add linux-image-generic-lts-xenial

Ubuntu arm64: While keeping kernel 4.2 for Trusty
(linux-image-generic-lts-trusty), add 4.4 backported from Xenial
(linux-image-generic-lts-xenial) to Armband ISO.

Xenial kernel 4.4 for Trusty has been tested with:
- AMD Softiron;
- APM Mustang;
- Cavium ThunderX;

While at it, remove obsoleted armband-rtc-efi-fix Debian package,
which was only needed on targets that are not compatible with 4.4,
and we plan on retiring soon.

v2->v3:
- keep 4.2 kernel instead of replacing it;
- selecting 4.4 kernel will be done via deploy settings, by
  overriding kernel flavor when calling `fuel-bootstrap`;
- dropped test changes, not needed when both 4.2 and 4.4 are in;

TODO:
- switch default kernel flavor to 4.4 when all our PODs are ready;

Closes: ARMBAND-48
Change-Id: I37600f12dd01facbd552f1d2d721bc7a3e0e2750
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>