meta-netboot: Add SELinux support

meta-netboot: Add SELinux support

Changes:
- Add logic to init.sh to handle autorelabeling the root filesystem
  if it is tagged as requiring it.  In practice this should only be
  required when booting images built on hosts that have xattr support,
  or if relabeling is manually triggered on subsequent boots.  There
  was a stated goal of avoiding the reboot from the later userspace
  handling of autorelabeling in CI, so this hook aims to enable that
  for the non-xattr build host corner case.
- Add a resolv-conf-relabel recipe that installs a systemd unit to
  relabel /etc/resolv.conf when netbooting.  This is required because
  the file always gets modified inside init.sh, and it does not seem
  worthwhile to always do the extra policy load required to relabel it
  in the initramfs.  This may need to be revisited if it proves
  difficult to handle denials in early booting when netbooting.
- Add the required extra packages for relabeling to IMAGE_INSTALL.

Bug-AGL: SPEC-4332

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I26b27c1cea68a029264352bd206c160cac3d451e
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/27791
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>