Support multiple paths in FAB_PLAN_INLUDE_PATH

I guess this is a poor version of overlays. Allowing two items in FAB_PLAN_INCLUDE_PATH means that the end-user can write a plan snippet which overrides something from within $(FAB_PATH)/common/plans, without needing to duplicating the whole tree (and keep it up to date).

My use-case for this is actually to remove a single package (udhcpd) from the default set which is otherwise OK.