Install kernel package via the meta package

Now that we have a single 4.14 for all systems and linux-latest in
apertis, install the kernel packages via the standard meta-packages
rather then specific versions.

Generally the debian meta for each architectures is linux-image-<debian
architecture name>. Only armhf is a bit different as the standard kernel
is armmp (arm multi-platform) so that needs special-casing.

In ostree-commit also split out the u-boot-imx handling; that really
shouldn't be there for *generic* arm ostree updates as it's a platform
specific u-boot package.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Denis Pynkin <denis.pynkin@collabora.com>
Differential Revision: https://phabricator.apertis.org/D7859