not-yet-updated drivers to at least function.

I want to get to the point where any header in include/ reflects a
standard user-level header (e.g. a POSIX header), while everything that's
specific to gPXE lives in include/gpxe/.  Headers that reflect a Linux
header (e.g. if_ether.h) should also be in include/gpxe/, with the same
name as the Linux header and, preferably, the same names used for the
definitions.

kernel.

Removed obsolete struct arprequest from if_arp.h and put it in nic.c so
that nic.c will still compile.  ARP will very shortly be handled by
net/arp.c instead.

to nic.c

initial device, and will also allow for e.g. a device menu to be presented
to the user.

Tie into new boot device framework.