main/mariadb: set CMake build type to MinSizeRel
It would be desirable to compile mariadb with the None build type to make it pick up our CFLAGS/CXXFLAGS from /etc/abuild.conf. Unfortunately, mariadb fails to build with the None build type. For this reason, we use the MinSizeRel build type as a compromise as it is closet to our default /etc/abuild.conf (i.e. uses -Os).