base-files: add support for rootfs compress from cmdline

Add support for configuring rootfs compression option from cmdline.

To enable rootfs compression declare in the cmdline
rootfs_compress_algorithm followed by the algorithm.

Example usage:
"... rootfs_compress_algorithm=zstd ..."

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

package/base-files/files/lib/preinit/80_mount_root

@@ -15,7 +15,12 @@ missing_lines() {
 }
 
 do_mount_root() {
-	mount_root
+	local rootfs_compression
+
+	rootfs_compression="$(cmdline_get_var rootfs_compress_algorithm)"
+
+	mount_root start rootfs_compress_algorithm="$rootfs_compression"
+
 	boot_run_hook preinit_mount_root
 	[ -f /sysupgrade.tgz -o -f /tmp/sysupgrade.tar ] && {
 		echo "- config restore -"