checkpatch: add check for keyword 'boolean' in Kconfig definitions

checkpatch: add check for keyword 'boolean' in Kconfig definitions

Discourage the use of keyword 'boolean' for type definition attributes of
config options as support for it will be dropped later on.

See http://lkml.kernel.org/r/cover.1418003065[email protected]

Signed-off-by: Christoph Jaeger <[email protected]>
Suggested-by: Daniel Borkmann <[email protected]>
Cc: Joe Perches <[email protected]>
Acked-by: Paul Bolle <[email protected]>
Tested-by: Paul Bolle <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>