Mariadb memory tuning should respect device total RAM

Currently, we provide one static my.cnf mariadb config for the mysqld server.

There are some hardcoded values that should be tuned according to a device total available RAM. i.e. on a 512 MB MOX, there should be lesser limits than on a 2 GB Omnia.

Also, mysqld parameters should be tunable by a user, preferably through uci config.