initramfs: rename gen_initramfs_list.sh to gen_initramfs.sh

The comments in usr/Makefile wrongly refer to the script name (twice).

Line 37:
    # The dependency list is generated by gen_initramfs.sh -l

Line 54:
    # 4) Arguments to gen_initramfs.sh changes

There does not exist such a script.

I was going to fix the comments, but after some consideration, I thought
"gen_initramfs.sh" would be more suitable than "gen_initramfs_list.sh"
because it generates an initramfs image in the common usage.

The script generates a list that can be fed to gen_init_cpio only when
it is directly run without -o or -l option.

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