net: fastboot: make UDP port net: configurable

The fastboot protocol uses per default the UDP port 5554. In some cases
it might be needed to change the used port. The fastboot utility provides
a way to specifiy an other port number to use already.

  fastboot -s udp:192.168.1.76:1234 boot fastboot.img

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>