bcache-tools: don't use probe-bcache and bcache-register

This allows us not to include any additional binaries for bcache
support in initrd; just an udev rule.
stage-1: add bcache support

This adds necessary udev rule into initrd to support autodetecting
bcache partitions in stage-1. It also fixes 'sh' path for the new
rule to work.
Merge pull request #4742 from abbradar/bcache-stage1

Support bcache volumes in initrd