boulder: Add user-customisable buildroot package install set to improve packager UX

[ermo]

The motivation for this is that we want the roots that get installed on package build to be as small as possible.

The conflicting motivation is that we also want to offer a good packaging experience when troubleshooting, which also includes having a decent amount of packages in the boulder chroot when using packaging %break_continue or %break_exit breakpoints to be dropped to a chroot shell.

This issue proposes two improvements whenever boulder spots a breakpoint or is invoked with the chroot argument:

• Add the user ~/.gitconfig to the root of the build root. This will make it easier to do the git init && git add . && git commit -m "Initial commit" dance and have the author and e-mail be pulled from ~/.gitconfig.
• Add the ability to specify a list of additions to the boulder chroot package set, to be installed whenever a recipe breakpoint is added. I am unsure if we can make this work well with an after-the-fact boulder chroot command, so it is possible that we may have to accept that this additional user-controlled set of packages is always installed if the user package set config file is present.