config: add categories support

Signed-off-by: Micky Ching <mickyching@gmail.com>
posts: move posts to each category

Signed-off-by: Micky Ching <mickyching@gmail.com>
posts: kernel: add mdev introduction

Signed-off-by: Micky Ching <mickyching@gmail.com>
posts: kernel: add block events introduction

Signed-off-by: Micky Ching <mickyching@gmail.com>