Standardize bootp, tftpboot, rarpboot, dhcp, and nfs command descriptions

cmd_net.c command descriptions were updated to describe the optional
hostIPaddr argument.  The dhcp command help message was also updated
to more closely reflect the other commands in cmd_net.c

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>