netloop: speed up NetLoop

NetLoop polls every cycle with getenv some environment variables.
This is horribly slow, especially when the environment is big.

This patch reads only the environment variables in NetLoop,
when they were changed.

Also moved the init part of the NetLoop function in a seperate
function.

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>