[gta02-core] Board parameters update

[Werner Almesberger]

Here's an update of the number of components we'll need and the structure
of the PCB:

- PCB size is about 10 x 6 cm2.
- PCB has 8 layers, 1 mm overall thickness. (*)
- Minimum trace width is 4 mil, with a 4 mil clearance between traces
  and between traces and pads.
- Surface finish is ENIG
- For the board structure, we need input from the PCB factory. Openmoko
  used a core of FR4 to which layers of PP 1080 65% and PP 1078 61% were
  added.
- PCB has components on both sides, about 10% on one, 90% on the other.
- Total number of components on the PCB will be about 400.
- Approximately 140 of the components on the PCB are unique.
- There are about 20 components that don't have a standard chip
  package (connectors, buttons, shields, etc.)
- Largest components are a 27 x 18 mm2 SIM/SD combi-cardholder, and a
  22 x 22 x 3 mm3 BGA module.
- Largest chip is a 14 x 14 mm2, 0.5 mm pitch 332-FBGA (SC32442)
- Smallest component is 0402.

(*) I'd like to try to make this work with only 6 layers, but I'm not
    sure if that's possible. Our circuit is considerably simpler than
    Openmoko's, with a greatly simplified memory bus and GSM in a
    module, so there is hope.

- Werner