Add debian repo generation and signing targets

- Add make-repo -- uses reprepro to generate repository metadata from
our generated packages
- Add sign-repo -- If the escalated_sign script exists, sign the Release
metadata, otherwise no-op
- Mark lots of targets as NONPARALLEL: they all touch the kernel's
Kbuild and so shouldn't run at the same time

Change-Id: Iaf307cc18961bad403290cdbb542f3e223430347
6 files changed