Allow config files and fonts to be on the memstick

With this, the following happens:

 * If you have a file PSP/flash0/font/ltn1.pgf under memstick/, it overrides the default flash0 font.  On Android, this should allow overriding fonts.  Fixes #1630.
 * The default config file location has changed to PSP/SYSTEM/ppsspp.ini.  If you have a ppsspp.ini in the old location, it will still be used just as before.  Fixes #868.
 * Resetting settings to defaults now deletes the active configuration file.  This allows you to "switch" to the memstick path.  It also applies when started with `--config=...` etc.

This overlaps slightly with #4070, but that's trying to solve a different problem.  With these changes, I'm primarily concerned with non-Windows.  I just saw now that it was Config/ not SYSTEM/, I don't care which... my PSP has a SYSTEM/ by default, so it seemed reasonable, but maybe the overlap is confusing.

-[Unknown]