uboot/Makefile move BINDIR

Set BINDIR before we include bsd.init.mk
so we can override it via local.init.mk

Reviewed by: imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D44413

(cherry picked from commit 01f3abbfcd59848e37118e5e2b868a7b3e98cc62)