Makefile: Keep list of exported variables

This can be useful to unexport them later.

Change-Id: I2ce9eff32d817ec190441550116376843abd1c11
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/21162
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
2 files changed