* APKBUILD: Leverage default_prepare and use builddir (no _).
* APKBUILD: Introduce some constants for better DRY-ness,
reformat package() function, create there a workdir
(otherwise buildbot won't start).
* buildmaster.initd: Put openrc-run in shebang.
Add create command, which is a wrapper around
`buildbot create-master -r $BASEDIR`.
It has not been truly tested, but at least it starts now if you perform
following command before starting buildmaster:
/etc/init.d/buildmaster create
