pmb/data/merge-usr: fix cannot overwrite error (MR 2388)

Don't just run one "mv" command to move everything from source to
target. This fails if the same directory already exists in target.
Instead, go into each directory in source, and move all files inside
that directory into the target directory structure.

Fix the following error:
  mv: cannot overwrite '/home/user/.local/var/pmbootstrap/chroot_native/usr/lib/sysctl.d': Directory not empty

pmb/data/merge-usr.sh

@@ -17,12 +17,41 @@ if [ -L "$CHROOT"/bin ]; then
 	exit 1
 fi
 
+dir_is_empty() {
+	local dir="$1"
+	local i
+
+	for i in "$dir"/*; do
+		if [ "$i" = "$dir/*" ]; then
+			return 0
+		else
+			return 1
+		fi
+	done
+}
+
 merge() {
 	local src="$1"
 	local dest="$2"
+	local dir
+
+	cd "$CHROOT/$src"
+
+	for dir in $(find . -type d | sort -r); do
+		mkdir -p "$CHROOT/$dest/$dir"
+
+		if ! dir_is_empty "$dir"; then
+			mv "$dir"/* "$CHROOT/$dest/$dir"
+		fi
+
+		if [ "$dir" != "." ]; then
+			rmdir "$dir"
+		fi
+	done
+
+	cd "$CHROOT"
+	rmdir "$src"
 
-	mv "$CHROOT/$src/"* "$CHROOT/$dest/"
-	rmdir "$CHROOT/$src"
 	ln -s "/$dest" "$CHROOT/$src"
 }