manibuilder: Move environment overrides into new Makefile.env

Change-Id: I7e1db64ab04c99a472770123865589278778e5f3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73775
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
2 files changed