initramfs: do not show compression mode choice if INITRAMFS_SOURCE is empty

Since commit ddd09bcc ("initramfs: make compression options not
depend on INITRAMFS_SOURCE"), Kconfig asks the compression mode for
the built-in initramfs regardless of INITRAMFS_SOURCE.

It is technically simpler, but pointless from a UI perspective,
Linus says [1].

When INITRAMFS_SOURCE is empty, usr/Makefile creates a tiny default
cpio, which is so small that nobody cares about the compression.

This commit hides the Kconfig choice in that case. The default cpio
is embedded without compression, which was the original behavior.

[1]: https://lkml.org/lkml/2020/2/1/160



Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>