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

test/test_frontend.py

@@ -0,0 +1,38 @@
+"""
+Copyright 2018 Oliver Smith
+
+This file is part of pmbootstrap.
+
+pmbootstrap is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+pmbootstrap is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
+"""
+import os
+import sys
+import pytest
+
+# Import from parent directory
+sys.path.append(os.path.realpath(
+    os.path.join(os.path.dirname(__file__) + "/..")))
+import pmb.config
+import pmb.parse
+import pmb.helpers.frontend
+import pmb.helpers.logging
+
+
+def test_build_src_invalid_path():
+    sys.argv = ["pmbootstrap.py", "build", "--src=/invalidpath", "hello-world"]
+    args = pmb.parse.arguments()
+
+    with pytest.raises(RuntimeError) as e:
+        pmb.helpers.frontend.build(args)
+    assert str(e.value).startswith("Invalid path specified for --src:")