vboot: merge different FWLIB_SRC versions into one

Since we are only building one fwlib library now, merge
FWLIB_SRCS, FWLIB2X_SRCS, and FWLIB_20_SRCS into one.
Remove the corresponding OBJS variables appropriately.

BUG=b:124141368, chromium:968464
TEST=make clean && make runtests
BRANCH=none

Change-Id: I190f3ca99ca67063fe0013dd490ef8869cd8f57c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1969258
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
1 file changed