rootfs/image: introduce new hook custom_apt_repo() (hashed into rootfs version); deploy different repo components/custom repos depending on rootfs or image

- rationale: we don't want an eternal chicken-egg problem with rootfs vs repo.
  - but, desktop rootfs require some parts of repo. case in point: `system-monitoring-center`
    - so only add certain components of repo (-desktop, -utils) to rootfs so that is honored
- introduce `custom_apt_repo()` hook for extensions to add their repos as well
  - same chicken-egg-avoiding is possible via param `CUSTOM_REPO_WHEN`

lib/functions/rootfs/rootfs-create.sh

@@ -145,8 +145,9 @@ function create_new_rootfs_cache_via_debootstrap() {
 		chroot_sdcard LC_ALL=C LANG=C setupcon --save --force
 	fi
 
-	# stage: create apt-get sources list (basic Debian/Ubuntu apt sources, no external nor PPAS)
-	create_sources_list "$RELEASE" "$SDCARD/"
+	# stage: create apt-get sources list (basic Debian/Ubuntu apt sources, no external nor PPAS).
+	# for the Armbian repo, only the components which are _not_ produced by armbian/build are included (-desktop and -utils)
+	create_sources_list_and_deploy_repo_key "root" "$RELEASE" "$SDCARD/"
 
 	# optionally add armhf arhitecture to arm64, if asked to do so.
 	if [[ "a${ARMHF_ARCH}" == "ayes" ]]; then