[REF] mrp: switch BoM line type to checkbox

For usability purposes, BoM line type (seleciton) has been replaced by a checkbox.
This change is pure usability, everything should continue to work as
before.

Field renaming: mrp.bom.line -> selection field `type` becomes boolean field `is_phantom`
Mapping for migration: ('normal', 'phantom') -> (False, True)