rootfs-create: move armbian-plymouth-theme back to distro-agnostic.sh

Restore the pre-branch install location for armbian-plymouth-theme:
post-cache in distro-agnostic.sh, alongside the 'if PLYMOUTH != yes
then apt-get remove plymouth' branch. That keeps both halves of the
PLYMOUTH toggle in one spot and stops straddling the rootfs-create
→ distro-agnostic boundary.

Previously moved to rootfs-create.sh (before the rootfs cache) so
that module_desktops install could apt-install armbian-plymouth-theme
from the just-populated apt source. The configng side is changing
in lockstep: when mode=build, module_desktops no longer tries to
install armbian-plymouth-theme — the build framework installs the
locally-built .deb artifact directly instead, here in
distro-agnostic.sh.

Author: igorpecovnik

lib/functions/rootfs/distro-agnostic.sh

@@ -413,11 +413,10 @@ function install_distribution_agnostic() {
 		fi
 	fi
 
-	# armbian-plymouth-theme is now installed in rootfs-create.sh (before
-	# cache) so it's available when module_desktops install runs. The
-	# non-plymouth path (remove plymouth) still runs here since it only
-	# matters on cached images that had plymouth pulled in transitively.
-	if [[ $PLYMOUTH != yes ]]; then
+	# install armbian-plymouth-theme
+	if [[ $PLYMOUTH == yes ]]; then
+		install_artifact_deb_chroot "armbian-plymouth-theme"
+	else
 		chroot_sdcard_apt_get_remove --auto-remove plymouth 2>/dev/null || true
 	fi
 

lib/functions/rootfs/rootfs-create.sh

@@ -226,13 +226,6 @@ function create_new_rootfs_cache_via_debootstrap() {
 	run_host_command_logged rm -fv "${SDCARD}"/etc/resolv.conf
 	run_host_command_logged echo "nameserver $NAMESERVER" ">" "${SDCARD}"/etc/resolv.conf
 
-	# Install plymouth theme BEFORE the desktop install so module_desktops
-	# finds it already present (armbian-plymouth-theme is a build artifact,
-	# not an apt.armbian.com package — module_desktops would 404 on it).
-	if [[ $PLYMOUTH == yes ]]; then
-		install_artifact_deb_chroot "armbian-plymouth-theme"
-	fi
-
 	# Install desktop via armbian-config INSIDE rootfs-create (before the
 	# cache tarball is saved) so desktop packages are included in the
 	# rootfs cache. The Armbian apt repo was configured above by