pstore/zone,blk: Add console frontend support

Support backend for console. To enable console backend, just make
console_size be greater than 0 and a multiple of 4096.

Signed-off-by: WeiXiong Liao <liaoweixiong@allwinnertech.com>
Link: https://lore.kernel.org/lkml/20200511233229.27745-5-keescook@chromium.org/


Signed-off-by: Kees Cook <keescook@chromium.org>