83xx, kmeter: fix compile error

CONFIG_SYS_MALLOC_LEN is defined in the board config, and
the keymile-common.h, which collects common options used
by all keymile-boards. This results in a compile error
when compiling the kmeter1 board. So remove this define
in the board config file.

Signed-off-by: Heiko Schocher <hs@denx.de>