board: ge: bx50v3: remove redundant targets

This replaces TARGET_GE_B{4,6,8}50V3 with common TARGET_GE_BX50V3.
The boards are identified automatically at runtime.

Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Nandor Han <nandor.han@ge.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>