De-couple weston from postmarketos-base (#233)

Thanks, craftyguy!

- UI is selectable with `pmbootstrap init`. Currently only 'weston' and 'none' are options. It'll automatically pick up any new `postmarketos-ui-*` packages added at later dates.
- splits off weston packages install from postmarketos-base and puts
them into postmarketos-ui-weston. Also note that NO weston packages are
installed by "pmbootstrap install" by default unless the user selects a ui in the `init`
- configuration of weston is now in postmarketos-ui-weston.
- the demos have been spun off to `postmarketos-demos`, and `postmarketos-ui-weston` lists this package as a dependency.

pmb/helpers/ui.py

@@ -0,0 +1,33 @@
+"""
+Copyright 2017 Clayton Craft
+
+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 glob
+
+
+def list(args):
+    """
+    Get all UIs, for which aports are available
+    :returns: ["none", "postmarketos-ui-one", "psotmarketos-ui-two", ...]
+    """
+    ret = []
+    for path in glob.glob(args.aports + "/postmarketos-ui-*"):
+        ui = os.path.basename(path).split("-", 2)[2]
+        ret.append(ui)
+    ret.append('none')
+    return ret