shannon/INFERNO: fix special handling of environment configuration

Remove some INFERNO related #ifdef's from common environment code by
fixing the board configuration settings (add CONFIG_ENV_SECT_SIZE).

While we are at it, fix comment which incorrectly talks about 4 KB
environment size, while it's actually 0x4000 = 16 KiB.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Rolf Offermanns <rof@sysgo.de>