pmbootstrap build --src: override source for any package (#1210)

* New "pmbootstrap build --src=/local/source/path hello-world" syntax
* The local source path gets mounted inside the chroot
* From there, a copy of the source code gets created with rsync (so
  we can write into the source folder if necessary, for better
  compatibility with all kinds of APKBUILDs)
* After the aport gets copied into the chroot before building (as
  usually), we extend the APKBUILD with overrides to make it use
  mountpoint's source instead of downloading the package's source
  from the web as usually
* The package built with the local source gets _pYYYYMMDDHHMMSS
  appended to the pkgver
* linux-postmarketos-mainline: use $builddir, fix patch checksum