Fix #183 (short term fix)

This is a temporary fix for #183, so that things can
build cleanly until the issue is fixed properly.

If RIMESTATS_CONF_ENABLED is 0, rimestats.foo will always
read as 0, since RIMESTATS_ADD(foo) doesn't do anything
Provide a macro to read rimestats when enabled
Rename RIMESTATS_CONF_ON to RIMESTATS_CONF_ENABLED
Replace reads of rimestats with RIMESTATS_GET(x)
Fix 8051 port builds to play nicely with #173
Merge remote-tracking branch 'george/bug-fixes/rimestats' into travis-test
Merge remote-tracking branch 'george/bug-fixes/uip-conf-ipv6' into travis-test