Change the format of processed-schema* from yaml to json to speed up
validation. With json output, using xargs and appending the output won't
work since json has explicit list begin and end characters. Instead,
we pass the schema files as a list in a temp file.

The parsing time for the processed schema goes down from ~2sec to 70ms.
Also, 'make dtbs_check' becomes 33% faster.

Some error messages are affected by this change. For example, "True was
expected" becomes "... is not of type 'boolean'". The order of messages
is also changed.

Signed-off-by: Andrei Ziureaev <andrei.ziureaev@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>