build: image: support specifying mksquashfs4 parallel threads

By enabling multi-threading file reading, the squashfs rootfs build
process can be greatly accelerated. We use 4 as the default thread
number. This is the default value for squashfs4 tool 4.7 if we don't
append "-block-readers" or "-small-readers" options.

For more test results:
https://github.com/plougher/squashfs-tools/blob/4.7/Documentation/4.7/README

Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19019
Signed-off-by: Nick Hainke <[email protected]>

config/Config-images.in

@@ -160,6 +160,22 @@ menu "Target Images"
 			  Select squashfs block size, must be one of:
 			    4, 8, 16, 32, 64, 128, 256, 512, 1024
 
+		config TARGET_SQUASHFS_BLOCK_READERS
+			int "mksquashfs tool parallel block reader threads"
+			depends on TARGET_ROOTFS_SQUASHFS
+			default 4
+			help
+			  Specify the number of parallel block reader threads
+			  (for files equal or larger than the squashfs block size).
+
+		config TARGET_SQUASHFS_SMALL_READERS
+			int "mksquashfs tool parallel small file reader threads"
+			depends on TARGET_ROOTFS_SQUASHFS
+			default 4
+			help
+			  Specify the number of parallel small file reader threads
+			  (for files less than the squashfs block size).
+
 	menuconfig TARGET_ROOTFS_UBIFS
 		bool "ubifs"
 		default y if USES_UBIFS

include/image.mk

@@ -86,6 +86,8 @@ SQUASHFS_BLOCKSIZE := $(CONFIG_TARGET_SQUASHFS_BLOCK_SIZE)k
 SQUASHFSOPT := -b $(SQUASHFS_BLOCKSIZE)
 SQUASHFSOPT += -p '/dev d 755 0 0' -p '/dev/console c 600 0 0 5 1'
 SQUASHFSOPT += $(if $(CONFIG_SELINUX),-xattrs,-no-xattrs)
+SQUASHFSOPT += -block-readers $(CONFIG_TARGET_SQUASHFS_BLOCK_READERS)
+SQUASHFSOPT += -small-readers $(CONFIG_TARGET_SQUASHFS_SMALL_READERS)
 SQUASHFSCOMP := gzip
 LZMA_XZ_OPTIONS := -Xpreset 9 -Xe -Xlc 0 -Xlp 2 -Xpb 2
 ifeq ($(CONFIG_SQUASHFS_XZ),y)