Move all upgrade scripts to a common directory

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.

package/gluon-core/files/etc/uci-defaults/zzz-gluon-upgrade

 #!/bin/sh
 
-
-UPGRADE_DIR=/lib/gluon/upgrade
-VERSION_DIR=/lib/gluon/version
-
-
-if [ -x /lib/gluon/legacy/legacy-upgrade ] && /lib/gluon/legacy/legacy-upgrade; then
-	HAS_LEGACY=1
-else
-	HAS_LEGACY=
-fi
-
-mkdir -p "$VERSION_DIR"
-
-
-version_of() {
-	opkg status "gluon-$1" | awk '/^Version: / { print $2 }'
-}
-
-oldversion_of() {
-	cat "$VERSION_DIR"/"$1" 2>/dev/null
-}
-
-do_dir() {
-	[ -d "$1" ] || return
-
-	local s
-	for s in "$1"/*; do "$s"; done
-}
-
-do_component() {
-	local component="$1"
-	local version="$(version_of "$component")"
-	[ "$version" ] || continue
-
-	(
-		cd "$component"
-
-		local oldversion="$(oldversion_of "$component")"
-		if [ -z "$oldversion" ]; then
-			if [ "$HAS_LEGACY" ]; then
-				do_dir legacy
-			else
-				do_dir initial
-			fi
-		fi
-
-		do_dir invariant
-
-		echo "$version" > "$VERSION_DIR"/"$component"
-	)
-}
-
-
-cd "$UPGRADE_DIR"
-
-do_component core
-
-for component in *; do
-	[ "$component" != 'core' ] || continue
-	do_component "$component"
+for script in /lib/gluon/upgrade/*; do
+	"$script"
 done
-
-if [ "$HAS_LEGACY" ]; then
-	/lib/gluon/legacy/legacy-upgrade-late
-fi

package/gluon-core/files/lib/upgrade/keep.d/gluon

-/lib/gluon/version/
-
 /lib/gluon/core/sysconfig/