Rework Linux Kernel fragment applications

Rework Linux Kernel fragment applications

- All kernel recipes must inherit linux-agl.inc
- All kernel recipes for a given version (e.g. 4.9) should include
  linux-VER-agl.inc in order to avoid duplication (such as the SMACK
  label patch or security fixes)
- We enforce having merge_config.sh be invoked at the end, in order to
  make sure fragments will be applied.
- Add a new fragment for Raspberry Pi to ensure the Pi Touchscreen is
  still enabled.

With all of the above, we fix a number of minor issues as well, such as
platforms manually enabling NBD/RAMDISK for netboot as the previous
logic was not working in all cases.

Bug-AGL: SPEC-946
Change-Id: Ic688e899df5861d83712af12d8e1c6c3c9643300
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12063
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: José Bollo <jobol@nonadev.net>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>