colibri-imx6ull: improve env badblock management

Use the complete 512kb (4 blocks) nand partition reserved for u-boot
environment instead of just the first block, this allows the module to
have a working environment even if 3 blocks are bad.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>