Makefile: Fix MAKEOBJDIRPREFIX command-line variable check for bmake

Unlike the old fmake, running make FOO=bar when using bmake doesn't put
FOO=bar in .MAKEFLAGS at the top level, it instead just puts FOO in
.MAKEOVERRIDES and the full MAKEFLAGS will be formed for sub-makes.
Moreover, this only applies for sub-makes in rules, so this doesn't
apply to those in shell assignments. This means that the current check
does not catch make MAKEOBJDIRPREFIX=..., only those defined in config
files. Thus we must also check .MAKEOVERRIDES explicitly.

Reviewed by: sjg
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31015