This implements the rewriting of a board's serial number.

Conflicts:
	software/lib/Makefile
	software/lib/liboattnusb.c
	software/lib/liboattnusb.h
	software/lib/oautest

which explains how to **really** change the eeprom serial number.

Now, everything is named NAME = oattnusb in every place.
The famous self-link allowing #include <product/product.h>
is also implied by this changeset

The value returned by the open_* routines, called 'a handle', is
nothing but an index into the static table of library contexts (one per
open device). The possibility that a handle is zero can be bothersome
for clients that assume it has some kind of file descriptor semantics.
So we make it start by 1 instead of 0 to avoid special, different
treatment between MagentaSYS attns and the USB type of attn.

Reported-by: Dimitris Lampridis <dimitrios.lampridis AT cern.ch>

This was all REVERSED in the original library, causing endless
confusion. Now, explicit ATT_MINUS_20dB labels are used, so no confusion
is possible.