meta-agl-core: Rework packagegroup-agl-core-boot

meta-agl-core: Rework packagegroup-agl-core-boot

To avoid the problem of it drift with respect to the upstream
packagegroup-core-boot that it was originally derived from, rework
packagegroup-agl-core-boot to just RDEPEND on it instead.  This
has the benefit of making the AGL additions more clearly visible.

Additionally:
- Define VIRTUAL-RUNTIME_base-utils-syslog for packagegroup-core-boot
  to match AGL's current default.
- Add a VIRTUAL-RUNTIME_rngd variable to allow downstream users to
  remove or replace rng-tools in their distro configuration.
- Explicitly add the packagegroup to IMAGE_INSTALL in the base
  agl-image-boot.inc to better match upstream core-image-minimal in
  definition, and make sure that any derived image that is created by
  including the inc file (instead of the bb file) is usable.

Bug-AGL: SPEC-3932

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ib82d5d6b561eca4d059c56b01a0caddefcb48776
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26376
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account