eNET: Fix saveenv crash

CONFIG_ENV_SIZE = CONFIG_ENV_SECT_SIZE = 128kB but CONFIG_SYS_STACK_SIZE
is only 32kB resulting in saveenv causing a stack overflow and crashing
U-Boot. Resolve by reducing CONFIG_ENV_SIZE to 4kB

Also fix up CONFIG_SYS_MALLOC_LEN to correctly use environment sector
size and add some comments to the memory organisation configuration

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>