configs: vcoreiii: Change CONFIG_ENV_SIZE

configs: vcoreiii: Change CONFIG_ENV_SIZE

Shrink the environment size for 3 reasons:

 - reading the environment it is slow, therefore having a smaller env
   improves the speed.
 - usually in the environment there are only few variables, therefore
   the enviromnent is almost empty.
 - because the same image can run on different boards which may have
   different flashes with different page sizes, the CONFIG_ENV_SECT_SIZE
   can't be change, it is set to least common multiple of the page sizes.

Adding this change improves the boot time. Before update for reading the
entire environment it took ~850 msec, after the change it takes ~40 msecs.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>