net: Make CMD_NET a menuconfig

Previously, CMD_NET was an alias for 2 commands (bootp and tftpboot) and
they we not able to be disabled. Separate out those 2 commands and move
CMD_NET up to the menu level, which more accurately represents the code.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Duncan Hare <dh@synoia.com>