Fix agl-appfw-smack and agl-netboot dependency issue

Fix agl-appfw-smack and agl-netboot dependency issue

Both layers need meta-python and meta-networking. But adding them to
BBLAYERS twice will cause a duplicate inclusion error.

Conditionals do not work out at this stage yet nicely, thus
we use an empty variable as indirection to only include it once.
(Kudos to Paul for the idea.)

Adapt bblayers.conf.sample, agl-appfw-smack and agl-netboot features.

This is the necessary follow-up to https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25844
and fixes the issue raised.

Bug-AGL: SPEC-3723
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Change-Id: I74e27b8a6e1f1a68ed370e01c2f2d406841873c8
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25881
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>