gbs local build support

gbs local build support

First version of localbuild support.
Design:
    As the quick start and first version, gbs-localbuild call build
    scripts directly, in other words, it's a wrapper of build scripts,
    Here's the basic work follow of gbs local build:
    1) Check options and generate command options transfer to 'build'
       i)   arch: build target arch
       ii)  repository: Specify package repositories to create build env
       iii) dist conf file: this can be specified from gbs.conf or
            command line
       iiii)buildroot: Specify build rootdir to setup chroot environment
    2) call linux32 to build 32bit packages if necessary
    3) change to root user to run rpmbuild
    4) call build scripts to all the build stuff.