Add a print command to fs_config_generator.py

This prints the uid map in a very simple format, with
AID_CONSTANT<space>uid
on each line.  This is super easy for other tools to
parse, and generate their own mappings, without requiring
edits to fs_config_generator.py itself.

Test: make, treehugger
Change-Id: I10e24ac29d440a24d43580880343d122ae1cdf02