buildman: Detect boards with multiple CONFIG_TARGETs defined

buildman: Detect boards with multiple CONFIG_TARGETs defined

The TARGET_xxx options are special in that they refer to a single target.
Exactly one should be enabled for each target, corresponding to a
defconfig file.

Detect configs which result in two TARGET_xxx options being set. For
example, at present, TARGET_POLEG and TARET_POLEG_EVB are enabled for the
same board.

Note: This warning is not displayed by default. An option will be added
to enable it.

Signed-off-by: Simon Glass <sjg@chromium.org>