linux: update paths to compiled cpio files

Linux has been using "usr/initramfs_data.cpio" for a few releases as the
generated cpio name, so the buildroot match of "...cpio.*" won't actually
clean out the previous result.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>