Split up fs_config_* modules per partition.

This was already done for partitions other than the system partition.
Add new, separate "_system" targets, and add new phony modules which
collects _system and _nonsystem targets for now.

Update the base_system.mk file to only depend on the system module, and
put the other ones in base_vendor.mk for now.

Bug: 80410283
Test: make fs_config_dirs[_*] fs_config_files[_*] and verify output
Change-Id: I7843578dbb9f27edf4c1e5b4a795a530f79e0be1