configs.py: Refine argparse to select build command type

To make configs.py more flexible, argparse is updated as below:
1. Use "--config_params" to list the build config variables of the
   input configuration.
2. Split the "build commands" into four types: "set_compiler",
   "cmake_config", "cmake_build" and "post_build". Only print the
   required type of commands.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I550eb6218fdac34d2aa634e730aba8ccfb05dd52
6 files changed