buildpackage: implement --[no-]build cmdline option

For enabling/disabling the builder script. The option is a bit
asymmetric: --git-no-build automatically disables the 'postbuild' hook
whereas --git-build does not enable it (e.g. if --git-no-hooks is
given).

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>