FROMLIST: kbuild: Enable armthumb BCJ filter for Thumb-2 kernel

xz_wrap.sh use $SRCARCH to detect the BCJ filter. However, assigning
arm BCJ filter to Thumb-2 kernel is not optimal. In my case, about 5%
decrease of image size is observed with armthumb BCJ filter:

Test results:
  hardware:      QEMU emulator version 3.1.0
  config:        vexpress_defconfig with THUMB2_KERNEL & KERNEL_XZ on
  arm BCJ:       4029808
  armthumb BCJ:  3827280

Choose armthumb BCJ filter for Thumb-2 kernel to make smaller images.

Signed-off-by: Jubin Zhong <zhongjubin@huawei.com>

Link: https://lore.kernel.org/lkml/1637323647-19988-1-git-send-email-zhongjubin@huawei.com/
Change-Id: I23c6e0fd2e54fdc84e78f122d6f4f06e54026696
Signed-off-by: Tao Huang <huangtao@rock-chips.com>

lib/decompress_unxz.c

@@ -129,8 +129,12 @@
 #	define XZ_DEC_POWERPC
 #endif
 #ifdef CONFIG_ARM
+#	ifdef CONFIG_THUMB2_KERNEL
+#		define XZ_DEC_ARMTHUMB
+#	else
 #		define XZ_DEC_ARM
 #	endif
+#endif
 #ifdef CONFIG_IA64
 #	define XZ_DEC_IA64
 #endif

scripts/xz_wrap.sh

@@ -9,6 +9,8 @@
 # You can do whatever you want with this file.
 #
 
+. include/config/auto.conf
+
 BCJ=
 LZMA2OPTS=
 
@@ -20,4 +22,8 @@ case $SRCARCH in
 	sparc)          BCJ=--sparc ;;
 esac
 
+if [ -n "${CONFIG_THUMB2_KERNEL}" ]; then
+	BCJ=--armthumb
+fi
+
 exec $XZ --check=crc32 $BCJ --lzma2=$LZMA2OPTS,dict=32MiB