support work_dir and buildconf options in gbs config, #1008

support work_dir and buildconf options in gbs config, #1008

work_dir is the topdir of all tizen source packages, and pre-built
build-conf git project will under that directory based on latest tizen
source code structure.

The new conf manager also support string interpolation, expecially for
work_dir, which can be used in specifying pre-built dir and build conf

build conf can be specified in gbs.conf now.

Examples of gbs.conf:

    [general]
    tmpdir=/var/tmp/export_dir/
    profile = profile.tizen2.1
    work_dir=~/local-source

    [repo.tizen2.1]
    url=${work_dir}/pre-built/

    [profile.tizen2.1]
    repos=repo.tizen2.1
    buildconf=${work_dir}/project-conf/build.conf

Structure of tizen source code

$ cd ~/local-source
$ tree .
  .
  |-- gbs-conf
  |   `-- gbs.conf
  |-- pre-built
  |-- project-conf
  |-- framework
  |-- ...
  `-- toolchains

Change-Id: Ia0e47500523dae09e90c0d4bee115e46628e3368