[gta02-core] new features in the BOM processing

[Werner Almesberger]

In line with the "what if" game, there's a now a third mode for the
BOM processing: in addition to "make optimist" and "make pessimist",
there's now a "make null" that generates an empty FIC/Openmoko Inc.
inventory.

To make it easier to add things from Digi-Key's catalog, I've written
a script that queries their Web site for the parts in digi-key.equ
and generates digi-key.inv and digi-key.dsc from this information.

Therefore, when running the BOM processing the next time, it will
query Digi-Key's Web site to build up the cache. The cache is
invalidated by editing digi-key.equ or by running "make clean" in
bom/dk/

Adding a Digi-Key part to our BOM is now simply a matter of

- adding a line to bom/dk/digi-key.equ to link Digi-Key part number
  and vendor name and vendor part number, and

- running "make optimist", "make pessimist", or "make null" to
  update the various stages in the BOM system

"make null" et al. print which parts can't be matched with anything
in an inventory. "make show-missing" shows the parts we know are
missing (listed in bom/fic/missing.inv). Finally, "make show-dk"
shows the "shopping list" generated for Digi-Key, with the current
price.

- Werner