[myri10ge] Add NonVolatile Option (nvo) support

Add NonVolatile Option (nvo) and NonVolatile Storage (nvs) support to
the myri10ge driver using the EEPROM read/write mechanism provided by
the NIC's Vendor Specific PCI capability.

The myri10ge NIC is capabile of storing 64KB or more of nonvolatile
options, but this patch advertises only 512 bytes of nvo storage
because iPXE malloc's a buffer matching the total size we advertise.
512 is plenty without wasting malloc'd memory.  (The 2 other drivers
currently supporting nvo advertise 256 bytes or less.)

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>