thie enables building it in a different directory, more
flexible install and configure options, and should make it
easier to eventualy create deb and rpm packages. Additionally
this makes it much easier to build and test this package
as it now behaves the same way as others we are using.
Move the rule to run a bit later, after
the standard udev rules have called blkid.
Don't run blkid manually, and run probe-bcache
as a fallback which can be patched out in a
package that depends on util-linux 2.24+.
probe-bcache: bail if libblkid finds anything
Preserve our last-resort safety check here
in case earlier rules fail to call blkid.
This means bcache devices will be registered earlier and in all
cases, not just when the rootfs fails to mount.
The initramfs hook is still there to ensure the bcache module
and udev rules are shipped if an initramfs is used.
