Add agl-package-management feature

Add agl-package-management feature

Changes:
- Move addition of package-management to IMAGE_FEATURES and
  associated package feed variables to a new agl-package-management
  feature template.  The feature is not included by default in
  the base set.
- To somewhat maintain the previous developer experience the new
  feature has been made a dependency of the agl-devel feature.
  In practice, the only images affected by this change are
  agl-image-minimal and agl-image-weston from meta-agl-core, the
  demo platform images in meta-agl have package management enabled
  by default.

Bug-AGL: SPEC-3971

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I56665e1cd82db19a697fdae2551a9276eab08961
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26393
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>