use squashfs with zstd as default, tested on Linksys NSLU2

mk/image.mk

@@ -200,7 +200,7 @@ ${FW_DIR}/${INITRAMFS}: ${STAGING_TARGET_DIR}/${INITRAMFS}_list
 
 ${BUILD_DIR}/root.squashfs: ${TARGET_DIR}/.adk
 	${STAGING_HOST_DIR}/usr/bin/mksquashfs ${TARGET_DIR} \
-		${BUILD_DIR}/root.squashfs -comp gzip \
+		${BUILD_DIR}/root.squashfs -comp zstd \
 		-nopad -noappend -root-owned $(MAKE_TRACE)
 
 ${FW_DIR}/${ROOTFSJFFS2}: ${TARGET_DIR}

target/linux/config/Config.in.fs

@@ -49,6 +49,9 @@ config ADK_LINUX_KERNEL_JFFS2_COMPRESSION_OPTIONS
 config ADK_LINUX_KERNEL_JFFS2_ZLIB
 	bool
 
+config ADK_LINUX_KERNEL_SQUASHFS_ZSTD
+	bool
+
 menu "Filesystems"
 
 config ADK_LINUX_KERNEL_EXT2_FS
@@ -249,6 +252,7 @@ config ADK_LINUX_KERNEL_JFFS2_FS
 
 config ADK_LINUX_KERNEL_SQUASHFS
 	tristate "SquashFS filesystem"
+	select ADK_LINUX_KERNEL_SQUASHFS_ZSTD
 	select ADK_LINUX_KERNEL_MISC_FILESYSTEMS
 	help
 	  Squashfs compressed read-only filesystem