drivers/net/e1000: Introduce CONFIG_E1000_NO_NVM

The e1000 driver expects to always have some kind of non-volatile memory
attached directly to the ethernet controller chip. This means that I would
have to add an additional separate flash chip to my custom board just to
store essentially the MAC address. Since I don't want to do that, this patch
introduces a new config option CONFIG_E1000_NO_NVM. If defined it disables
all accesses to the NVM. I have tested the patch with a 82574 controller.

Signed-off-by: Rojhalat Ibrahim <imr@rtschenk.de>