Adds mysql-common element

This element contains code shared by mysql and mariadb elements.

Change-Id: I5edc6c2cebfa36d16cf3dcd110c2fe1488c9c3ab
Adds mysql-dev and mariadb-dev elements

It have to be separate elements because installation differs depending
whether mysql or mariadb is used.

Change-Id: I9299b8e21a5ea2ff13ab9f11d851703a67c6b130
Merge "Adds mysql-common element"
Merge "Adds mysql-dev and mariadb-dev elements"