libopie2: Enable SQL PIM backend.

libopie2: Enable SQL PIM backend.
* Let users store their data reliably. XML backend is still used by default,
user must activate SQLite in config files (see OPIE wiki).
* Regarding concerns due to which is wasn't enabled before: "too large for 16MB
flash devices" - such devices welcome to disable-override it; "no sync" - apparently,
no sync with SQLite backend active, but XML is till by default.
* SQLite backend and opie-pimconvert tested to work OK on hx4700 (on small dataset
though, more tetsing pending). Suprising;y, I had rather weird issues with sqlite3
on qemuarm.