initramfs: refactor the initramfs build rules

Currently, usr/gen_initramfs.sh takes care of all the use-cases:

 [1] generates a cpio file unless CONFIG_INITRAMFS_SOURCE points to
     a single cpio archive

 [2] If CONFIG_INITRAMFS_SOURCE is the path to a cpio archive,
     use it as-is.

 [3] Compress the cpio file according to CONFIG_INITRAMFS_COMPRESSION_*
     unless it is passed a compressed archive.

To simplify the script, move [2] and [3] to usr/Makefile.

If CONFIG_INITRAMFS_SOURCE is the path to a cpio archive, there is
no need to run this shell script.

For the cpio archive compression, you can re-use the rules from
scripts/Makefile.lib .

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>