vexpress64: store env in flash

vexpress64: store env in flash

Add support for storing the environment in CFI NOR flash on Juno and FVP
models.

I also removed some config values that are not used by CFI flash parts.

Juno has 1 flash part with 259 sectors.  The first 255 sectors are
0x40000 (256kb) and are followed by 4 sectors of 0x10000 (64KB).

FVP models simulate a 64MB NOR flash part at base address 0x0FFC0000.
This part has 256 x 256kb sectors.  We use the last sector to store the
environment.

To save the NOR flash to a file, the following parameters should be
passed to the model:

    -C bp.flashloader1.fname=${FILENAME}
    -C bp.flashloader1.fnameWrite=${FILENAME}

Foundation models don't simulate the NOR flash, but having NOR support
in the u-boot binary does not harm:  attempting to write to the NOR will
fail gracefully.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>