meta-agl-core: update packagegroup-agl-core-boot

meta-agl-core: update packagegroup-agl-core-boot

Update packagegroup-agl-core-boot to match the current state of
packagegroup-core-boot in upstream poky master.  The changes
remove use of some of the global VIRTUAL-RUNTIME variables that are
now assumed to be globally scoped, and replacing the hard-coded
busybox usage with appropriate VIRTUAL-RUNTIME variables.

Bug-AGL: SPEC-3819

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I5d16c44bf78afd2a8b0914c3381f94e7e5382223
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26099
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>