lib/Kconfig: use bool instead of boolean

lib/Kconfig: use bool instead of boolean

Keyword 'boolean' for type definition attributes is considered
deprecated and, therefore, should not be used anymore.

See http://lkml.kernel.org/r/cover.1418003065.git.cj@linux.com
See http://lkml.kernel.org/r/1419108071-11607-1-git-send-email-cj@linux.com

Signed-off-by: Christoph Jaeger <cj@linux.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>