patman: Parse checkpatch by message instead of by line

Parse each empty-line-delimited message separately. This saves having to
deal with all the different line content styles, we only care about the
header ERROR | WARNING | NOTE...

Also make checkpatch print line information for a uboot specific
warning.

Signed-off-by: Evan Benn <evanbenn@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>