Update the u-boot-rpi2 port to use the u-boot-master port.
This also bring u-boot to version 2017.09 and the firmware files are no
longer in this port but in sysutils/rpi-firmware.
Also I'm stepping in as maintainer.
Tested On: RPI2v1.1, RPI2v1.2
Reviewed by: imp
Approved by: imp
Differential Revision: https://reviews.freebsd.org/D13049