vboot_reference/Makefile: Fix building with USE_FLASHROM:=0

Unfortunately the USE_FLASHROM build flag broke during a refactor
in commit f0af257a9b333b. This flag is needed for coreboot builds.
Resolve by putting relevant files under guards.

BUG=b:221234560,b:220833585
BRANCH=none
TEST=`USE_FLASHROM={0,1} make`

Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Change-Id: Ic10a3909bf070897cef7f1556deef57eba6c57f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3493527
Tested-by: Edward O'Callaghan <quasisec@chromium.org>
Auto-Submit: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Commit-Queue: Martin Roth <martinroth@google.com>
1 file changed