M421 was not connected up for AUTO_BED_LEVELING_BILINEAR.
M421 needed to migrate mesh data to new UBL EEPROM layout.

As it turns out...   an unsigned int can not go to less than 0,  so the
loop never terminates.

The TB2 is a Indiegogo funded 3d printer. It uses a MKS SMELZI V1.0
main board (based on MELZI, i.e. Sanguinololu). I comes as a complete kit,
including display and panel.
Two variants exists, L10 and L16, with a height of 100 and 160 mm.
The heating function of the provided bed can enabled by soldering a
thermistor and some wires to it.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

The OLED is driven by an SSD1306, connected to the board via
I2C, the rotary encoder is connected to 3 GPIO pins.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

- Make all `unified_bed_leveling` data/methods static
- Move some UBL-related variables into the class
- Replace `map_[xy]_index_to_bed_location` with `mesh_index_to_[xy]pos`