fs/squashfs: Add option to compress with extreme options

mksquashfs allows to tweak many compressions parameters. Currently they
can't be changed from kmenu. Leaving out potential space savings.

This adds the option to enable a set of predetermined compression
options. This option is enabled by default for lz4 since lz4 currently
implicitly added the extreme to it in the makefile. So this aids in
keeping backward compatibility.

Signed-off-by: Linus Kaschulla <linus@cosmos-ink.net>
[yann.morin.1998@free.fr:
  - fix check-package
  - change the default code-style, add the legacy comment
  - always add the qstriped string, as it's empty when not used
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

fs/squashfs/Config.in

@@ -89,4 +89,30 @@ config BR2_TARGET_ROOTFS_SQUASHFS4_ZSTD
 
 endchoice
 
+config BR2_TARGET_ROOTFS_SQUASHFS_EXTREME_COMP
+	bool "extreme compression when available"
+	default y if BR2_TARGET_ROOTFS_SQUASHFS4_LZ4  # legacy
+	help
+	  Use options to increase compression ration as much as
+	  possible, like using architecture-specific options, at
+	  the cost of time when assembling the filesystem image.
+
+	  For example:
+	   - with gzip and lzo, use -Xcompression-level 9
+	   - with xz use arch-specific bcj (branch-call-jump) filters
+	   - with zstd use -Xcompression-level 22
+	   - and more
+
+config BR2_TARGET_ROOTFS_SQUASHFS_COMP_OPTS
+	string
+	default "-Xcompression-level 9" if BR2_TARGET_ROOTFS_SQUASHFS4_GZIP
+	default "-Xcompression-level 9" if BR2_TARGET_ROOTFS_SQUASHFS4_LZO
+	default "-Xhc" if BR2_TARGET_ROOTFS_SQUASHFS4_LZ4
+	default "-Xbcj arm,armthumb" if BR2_TARGET_ROOTFS_SQUASHFS4_XZ && (BR2_arm || BR_aarch64)
+	default "-Xbcj powerpc" if BR2_TARGET_ROOTFS_SQUASHFS4_XZ && (BR2_powerpc || BR2_powerpc64)
+	default "-Xbcj sparc" if BR2_TARGET_ROOTFS_SQUASHFS4_XZ && (BR2_sparc || BR2_sparc64)
+	default "-Xbcj x86" if BR2_TARGET_ROOTFS_SQUASHFS4_XZ && (BR2_i386 || BR2_x86_64)
+	default "-Xcompression-level 22" if BR2_TARGET_ROOTFS_SQUASHFS4_ZSTD
+	depends on BR2_TARGET_ROOTFS_SQUASHFS_EXTREME_COMP
+
 endif

fs/squashfs/squashfs.mk

@@ -9,14 +9,15 @@ ROOTFS_SQUASHFS_DEPENDENCIES = host-squashfs
 ROOTFS_SQUASHFS_ARGS = \
 	-noappend \
 	-processors $(PARALLEL_JOBS) \
-	-b $(call qstrip,$(BR2_TARGET_ROOTFS_SQUASHFS_BS))
+	-b $(call qstrip,$(BR2_TARGET_ROOTFS_SQUASHFS_BS)) \
+	$(call qstrip,$(BR2_TARGET_ROOTFS_SQUASHFS_COMP_OPTS))
 
 ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS_PAD),)
 ROOTFS_SQUASHFS_ARGS += -nopad
 endif
 
 ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_LZ4),y)
-ROOTFS_SQUASHFS_ARGS += -comp lz4 -Xhc
+ROOTFS_SQUASHFS_ARGS += -comp lz4
 else ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_LZO),y)
 ROOTFS_SQUASHFS_ARGS += -comp lzo
 else ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_LZMA),y)