checkox-touch: add option to embed local plainbox in get-libs

This patch makes get-libs accept --get-local-plainbox option that builds
plainbox from local tree and copies it to appropriate directories.

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1375586
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge by tarmac [r=sylvain-pineau][bug=1375586][author=kissiel]"