Update the u-boot-rpi 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: RPI-B, RPI0v1.2, RPI0v1.3
Reviewed by: imp
Approved by: imp
Differential Revision: https://reviews.freebsd.org/D13048