Fix 8051 port builds to play nicely with #173

#173 attempts to make the switch between IPv4/IPv6 exclusively through defines.

This change is undesirable for the Sensinode and CC2530dk ports, which require such knowledge at the build system in order to adjust the build accordingly, e.g. to turn code banking on or off if unspecified.

Before #173, the top level `Makefile.include` would notify the preprocessor through `CFLAGS += -DUIP_CONF_IPV6=1`, based on the value of the `UIP_CONF_IPV6` make variable.

Since IPv6 example makefiles still specify `UIP_CONF_IPV6=1`, we do in the platform makefiles what was used to be done in `Makefile.include`