mtd: bcm47xxpart: alternative MAGIC for board_data partition

Some devices (like WNDR3700v3) have board_data without MPFR magic, some
extra header or extra NVRAM around 0x100. In such case we have to look
for another magic which is BD 0B 0D BD (BD probably stands for Board
Data). It's located "far far away", so instead of extending buffer add
another mtd_read.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>