Support an overrides/patches/$srpm directory

[jlebon]

A major part of hacking on the OS is being able to test patches. Right now this is a very manual process with e.g. fedpkg local on the side and then copying into overrides/rpm (or sending them to an existing VM and doing override replace). It'd be cool if we could do e.g.:

$ mkdir -p overrides/patches/ignition
$ (cd overrides/patches/ignition && curl -L https://github.com/coreos/ignition-dracut/pull/60.patch)
$ cosa build
Building ignition locally with override patches...
...

This would: download the SRPM, unpack it, apply the patch, rpmdev-bumpspec, build, and add the built RPMs to the overrides/rpm dir. Would be easier to require the container image to be based on the same buildroot though so we don't have to use mock. This is the case for FCOS at least :).