board: MCR3000: Use smaller flash sector for environment

Latest versions of u-boot have increased in size and require more
than the 256kb allocated to it.

The MCR3000 board is equipped with an AM29LV160DB boot flash which
is organised as follows:
- One 16kb block
- Two 8kb block
- One 32kb block
- Thirty one 64kb blocks

At the time being, u-boot is a single piece occupying the 256 first
kbytes, then the environment is stored in the following 64kb block

The environment being quite tiny, we save one 64kb block by embedding
the environment in the first 8kb block, hence allowing to increase
the monitor size to 320kb.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>