loader: network code update

hi!

After pushing the network code update, there is review up: https://reviews.freebsd.org/D10631

For testing it, you would also like to increase the read sizes like:

tftp.blksize=“8196”
nfs.read_size=“4096"

for nfs, the 4k is currently max value, we probably would like to rise the allowed max to 8-16k.

rgds,
toomas