modules/generic_stage_target.py, modules/stage1_target.py: Add a target_mounts dictionary

modules/generic_stage_target.py, modules/stage1_target.py: Add a target_mounts dictionary

Temporary location to define TARGET_MOUNTS_DEFAULTS.
It will be moved to a new defaults.py file in a later commit.
I also plan to make them configurable.

Also:
* Clean up all self.mounts, self.mountmap usage.
* Replace multiple path additions with one instance at the
  beginning of the function, reuse the result multiple times.
* Add some extra debug prints (to be converted to logging later)