kbuild: clean compressed initramfs image

Since 9e3596b0 ("kbuild: initramfs cleanup, set target from Kconfig")
"make clean" leaves behind compressed initramfs images.  Example:

  $ make defconfig
  $ sed -i 's|CONFIG_INITRAMFS_SOURCE=""|CONFIG_INITRAMFS_SOURCE="/tmp/ir.cpio"|' .config
  $ make olddefconfig
  $ make -s
  $ make -s clean
  $ git clean -ndxf | grep initramfs
  Would remove usr/initramfs_data.cpio.gz

clean rules do not have CONFIG_* context so they do not know which
compression format was used.  Thus they don't know which files to delete.

Tell clean to delete all possible compression formats.

Once patched usr/initramfs_data.cpio.gz and friends are deleted by
"make clean".

Link: http://lkml.kernel.org/r/20190722063251.55541-1-gthelen@google.com
Fixes: 9e3596b0 ("kbuild: initramfs cleanup, set target from Kconfig")
Signed-off-by: Greg Thelen <gthelen@google.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

usr/Makefile

@@ -11,6 +11,9 @@ datafile_y = initramfs_data.cpio$(suffix_y)
 datafile_d_y = .$(datafile_y).d
 AFLAGS_initramfs_data.o += -DINITRAMFS_IMAGE="usr/$(datafile_y)"
 
+# clean rules do not have CONFIG_INITRAMFS_COMPRESSION.  So clean up after all
+# possible compression formats.
+clean-files += initramfs_data.cpio*
 
 # Generate builtin.o based on initramfs_data.o
 obj-$(CONFIG_BLK_DEV_INITRD) := initramfs_data.o