initramfs: generalize initramfs_data.xxx.S variants

Remove initramfs_data.{lzo,lzma,gz,bz2}.S variants and use a common
implementation in initramfs_data.S.  The common implementation expects the
file name of the initramfs to be defined in INITRAMFS_IMAGE.

Change the Makefile to set the INITRAMFS_IMAGE define symbol according
to the selected compression method.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Acked-by: Michal Marek <mmarek@suse.cz>
Acked-by: "H. Peter Anvin" <hpa@zytor.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>