At present we show when a driver is missing but this is not always that
useful. There are various reasons why a driver may appear to be missing,
such as a parse error in the source code or a missing field in the driver
declaration.

Update the implementation to record all warnings for each driver, showing
only those which relate to drivers that are actually used. This avoids
spamming the user with warnings related to a driver for a different board.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Walter Lozano <walter.lozano@collabora.com>