support incremental build

support incremental build

Incremental build is very important for developer, especially for developing
big project, such as webkit, kernel, ect.

This patch add three options to support incremental build:
--no-build: create build root using spec file, and don't execute
            build stage
--short-circuit: incremental build option, this need work with --stage
            option.
--no-topdir-cleanup: don't remove rpmbuild build topdir, this need
            to be specified for incremental build

Basic usage:
1) $ build test.spec --no-build
2) $ mount source code to buildroot/abuild/rpmbuild/BUILD/$name-$verson
3) build packages
 a) just compile
 $ build test.spec --no-init --short-circuit --no-topdir-cleanup --stage=bc
 b) generate RPM packags
 $ build test.spec --no-init --short-circuit --no-topdir-cleanup --stage=bs
4) $ umount buildroot/abuild/rpmbuild/BUILD/$name-$verson

Change-Id: I7e1f7029c49e85fdb3f5a260631c10d052164d17