db: create uvol database symlink to a configurable mountpoint

This allows apk to create a db-uvol symlink on any write operation if uvol is active, and the database does not exist yet. This allows relocation of the uvol-db to an alternative directory, but still support generic apk builds to find the uvol database using the symlink.

supercedes !109 (closed) and !166 (closed)

@aparcar, @ynezz, @dangowrt does this look something that works for you?