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>