sysutils/u-boot-tools: Add recently upstream patch to fix endianness checks

Patch originally sent upstream by jhibbits@; fix endianness checks so that
powerpc hosts can actually build powerpc images. Backport it to our 2017.09
since it won't appear in a release until 2018.03 at the earliest.

Reviewed by: manu
Approved by: imp (ports)
Differential Revision: https://reviews.freebsd.org/D14219