iSCSI hack.

This driver really needs to be rewritten.
It tries to build both ISA and PCI images,
and makes life diffifult for the build system
and rom-o-matic.net.  The warning was just a reminder
that it needs to be cleaned up and re-factored
to split the PCI and ISA drivers.

This ancient ISA driver should probably be removed.
It is hard to get a card to test it with, and there
are comments to the effect that it cannot work with
relocation.  I would be quite interested to get a
bug report by someone who actually has this card.

Kill off use of etherboot.h outside drivers/net.

safe dropping of the netdev ref by the driver while other refs still
exist.

Add netdev_irq() method.  Net device open()/close() methods should no
longer enable or disable IRQs.

Remove rx_quota; it wasn't used anywhere and added too much complexity
to implementing correct interrupt-masking behaviour in pxe_undi.c.