vboot: reduce references to parent's files in Makefiles

this change also allows vboot1 code to use flash as nvram device.

BUG=none
BRANCH=ToT
TEST=Built nyan_blazw using vboot1/2. Built falco, lumpy, nyan, blaze,
parrot, rambi, samus, storm, pinky with default configuration.

Original-Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Change-Id: Ie97a4436d4fc10851a535adfdb45c4d499e45b5d
Original-Reviewed-on: https://chromium-review.googlesource.com/229598
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>

(cherry picked from commit 8ac8ff28bab1337782e8694275bb2c644b86f38a)
Signed-off-by: Aaron Durbin <adurbin@chromium.org>

Change-Id: Ief087fedbf29b4b82c1668ad5603c121323dcbf2
Reviewed-on: http://review.coreboot.org/9440
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)
2 files changed