sysutils/u-boot-master: fix on powerpc64

Fixes build of u-boot ports:
binman: Error 1 running 'mkimage -d ./mkimage-in-simple-bin.mkimage-rockchip-tpl:./mkimage-in-simple-bin.mkimage-u-boot-spl -n rk3568 -T rksd ./idbloader.img': Error: image verification failed
mkimage: Failed to verify header of ./idbloader.img