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

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

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

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

While at it, remove obsoleted rtc-efi fixes.

Notes:
- keep 4.2 kernel inside the ISO;
- selecting 4.2 kernel may be done via deploy settings, by
  overriding kernel flavor when calling `fuel-bootstrap`;

Related-bug: ARMBAND-48

Change-Id: Ice1f95939f0d7d9491befd20030740850b1a0e61
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>