buildman: Allow conflicting tags to avoid spurious errors

Conflicting tags can prevent buildman from building two series which exist
one after the other in a branch. There is no reason not to allow this sort
of workflow with buildman, so ignore conflicting tags in buildman.

Change-Id: I2231d04d8684fe0f8fe77f8ea107e5899a3da5e8
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>