util/nvmutil: serious re-factoring (speed boost!)

word/setWord no longer mitigates endianness. instead,
all bytes are swapped after reading and before writing the
file, and only if the host is big endian

this improves performance on little endian hosts, which is
most machines, and the code is much simpler, so it's more
robust and less likely to break

mac address endianness made more clear in code, including
with a comment that explains it

(the nvm section contains little endian words, *except* the
mac address whose words are stored big endian)