kmsg: add function for adding and deleting additional buffers

kmsg: add function for adding and deleting additional buffers

Additional kmsg buffers should be created and deleted dynamically.

Adding two functions
* kmsg_sys_buffer_add() creates additional kmsg buffer returning minor
* kmsg_sys_buffer_del() deletes one based on provided minor

Change-Id: Ibb85543b830ecd186e8cf7ebdf560f72c0bba83c
Signed-off-by: Marcin Niesluchowski <m.niesluchow@samsung.com>
[Fixed bug that allow adding more buffers than available minors]
Signed-off-by: Kazimierz Krosman <k.krosman@samsung.com>