Some shuffling around of the image management code; this needs tidying up.

list.

Added FEATURE() macros to most relevant (non-driver) files.

and generate DHCP options to indicate features to DHCP server (and to
PXE NBPs).

iSCSI hack.

really ugly hack at present, but that doesn't hugely matter since I'm
aiming to change the interface to iSCSI devices anyway within the next
week.

etherboot.h is only now used by drivers that have not been updated to
the new API.

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

SCSI interface not yet implemented.

implemented; should be done at some point.)

applied.

xfer_window() function, which can return a scalar rather than a
boolean.

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.

not apply DHCP options in dhcp_configure_netdev().

posix_io.c and this PXE error makes more sense in this context.