gen_initramfs.sh: use gen_init_cpio -o parameter

gen_init_cpio can now write to a file directly, so use it when
gen_initramfs.sh is called with -o (e.g. usr/Makefile invocation).

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250819032607.28727-6-ddiss@suse.de


Signed-off-by: Nathan Chancellor <nathan@kernel.org>