[make] Compare whole words for LKINC check. (#160)

Previously, if lk was built from an external directory that contained
"lk" in the name, the "findstring" check would succeed and not add lk's
directory to LKINC. This switches to a word comparison instead of
a substring search.

makefile

@@ -10,7 +10,7 @@ DEFAULT_PROJECT ?=
 TOOLCHAIN_PREFIX ?=
 
 # check if LKROOT is already a part of LKINC list and add it only if it is not
-ifneq ($(findstring $(LKROOT),$(LKINC)), $(LKROOT))
+ifeq ($(filter $(LKROOT),$(LKINC)), )
 LKINC := $(LKROOT) $(LKINC)
 endif