Include the jedec "extended device information string" in the criteria used
to match a chip to our table of metadata describing the chips. At least one
new DataFlash chip has a 3-byte jedec ID identical to its predecessors and
differs only in the extended info, and it has different metadata requiring a
unique entry in the table. This paves the way for supporting such chips.

The metadata table now includes two new fields, extmask and extid. The two
bytes of extended info obtained from the chip are ANDed with extmask then
compared to extid, so it's possible to use only a subset of the extended
info in the matching.

We now always read 6 bytes of jedec ID info. Most chips don't return any
extended info, and the values read back for those two bytes may be
indeterminate, but such chips have extmask and extid values of 0x0000 in the
table, so the extid effectively doesn't participate in the matching on those
chips and it doesn't matter what they return in the extended info bytes.