board: ge: bx50v3: add missing partnum variable

Add missing ${partnum} to set rootdev correctly when
booting from USB or MMC.

Signed-off-by: Kimmo Surakka <kimmo.surakka@ge.com>
[Rebased against v2016.05 and adjusted the variable name]
Signed-off-by: Andrew Shadura <andrew.shadura@collabora.co.uk>