2lib: create generic return code for unimplemented external functions

vboot_reference has a set of "external" functions which are
defined by the client and linked in at compile time.

Instead of creating a separate return code for each unimplemented
external function, group them all into one.

BUG=chromium:944804
TEST=make clean && make runtests
BRANCH=None

Change-Id: Ic3896572c71aabf99590758c7fc3848b61f9e078
Reviewed-on: https://chromium-review.googlesource.com/1539433
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Joel Kitching <kitching@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
2 files changed