kconfig: add a debug option to print defaults in menuconfig

This adds a debug option to print defaults and their conditions
inside config description under menuconfig/xconfig. This aids in
debugging of kconfig creation.

Change-Id: Ie651435f6b2115dac1bd3a6f7a1b3c2df3c9b0ed
Signed-off-by: Daniel Leung <daniel.leung@intel.com>