doc: Update general information.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
build: Update list of exported symbols.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
build: Add check make target.

Which just runs check_symbols.sh script which looks if there are any
symbols exported by mistake.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>