package/mariadb: add option to disable build of embedded server

Size of output/target/usr:

with embedded: 1,7G
without embedded: 648M

This config option saves space on the target if the embedded server
is not used by any other package:

https://mariadb.com/kb/en/library/embedded-mariadb-interface/


Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Ryan Coe <bluemrp9@gmail.com>
[Peter: make Config.in option depend on _MARIADB_SERVER]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>