toradex: config block handling

Add Toradex factory configuration block handling. The config block is a
data structure which gets stored to flash during production testing. The
structure holds such information as board resp. hardware revision,
product ID and serial number which is used as the NIC part of the
Ethernet MAC address as well. The config block will be read upon boot by
the show_board_info() function, displayed as part of the board
information and passed to Linux via device tree or ATAGs.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>