Lift userboot's devicename.c as a starting point for BERI's loader --
it seems the most suitable of the various copy-and-paste options.
Complete lifting of userboot's devicename.c with BERI renaming and
compile-related fixes.
Begin to make beri_disk.c more real.
Checkpoint in-progress path to storage drivers being hooked up to BERI's
version of loader: fork beri_disk.c into two parts, one for each of our
two storage drivers (CFI, SD card); extract information passed from boot2
relating to where we were loaded from; use devicename.c's implementations
of device-name parsing.  Quite a lot more to do here as we flesh out the
abstractions above the microdrivers (which will need some new methods as
well).