Makefile:  Be sure keyring mtime is updated by recipe

keyring.gpg recipe is run if prereqs change, but gpg won't touch it if
the keys didn't change.  keyring.gpg is a prereq for other targets, so
it needs to be touched.
Build kernel packages against locally-built runtime packages

Kernel packages depend on Xenomai runtime packages for the Xenomai
patch.

The Xenomai runtime packages were being retrieved from the
machinekit.com Debian archive, despite the brand-new local packages
built earlier in the make run.  This doesn't make sense: the kernel
packages should be built against matching runtime packages and
deployed to an archive together.

- After runtime package build:
  - New target builds local PPA
  - Updates pbuilder chroot tarball with PPA
- PPA directory is passed into pbuilder config for kernel build
Updates to README