Netfs support for EFI platform

[Bean]

Hi,

r1825 adds netfs support for EFI, which allows you to netboot or read
files from tftp server on EFI/UEFI platform.

First you need to setup a server. For intel mac, you need BSDP server.
for UEFI, normal DHCP server might work, although I have yet to found
a UEFI machine that's capable of booting from network. You also need
tftp server for file transfer. tftp server should use "-s" or similar
option to skip the fs path such as "/tftpboot/".

Put burg64.efi to /tftpboot/, and construct burg.cfg, then configure
netboot server to use burg64.efi as boot file.

Inside burg, network device is (net0), (net1), etc. For example, this
dumps the content of burg.cfg on net0:

cat (net0)/burg.cfg

When booting from network, (net0) is always the device you boot from,
prefix is also set to (net0).

In the intel macs I tested, you can even use (net0) device to read
files from tftp server when booting from local disk.

--
Bean