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

pmb/parse/arguments.py

@@ -314,6 +314,11 @@ def arguments():
                        " necessary")
     build.add_argument("--strict", action="store_true", help="(slower) zap and install only"
                        " required depends when building, to detect dependency errors")
+    build.add_argument("--src", help="override source used to build the"
+                       " package with a local folder (the APKBUILD must"
+                       " expect the source to be in $builddir, so you might"
+                       " need to adjust it)",
+                       nargs=1)
     build.add_argument("-i", "--ignore-depends", action="store_true",
                        help="only build and install makedepends from an"
                        " APKBUILD, ignore the depends (old behavior). This is"