[make] rename INCLUDES -> GLOBAL_INCLUDES

To be more consistent, rename make variable INCLUDES to GLOBAL_INCLUDES.
Also remove the need to put -I in front of each field, the make system
will do that for you.

To fix your module makefiles:
-Change INCLUDES -> GLOBAL_INCLUDES
-Remove -I prefix

app/tests/rules.mk

@@ -2,7 +2,7 @@ LOCAL_DIR := $(GET_LOCAL_DIR)
 
 MODULE := $(LOCAL_DIR)
 
-INCLUDES += -I$(LOCAL_DIR)/include
+GLOBAL_INCLUDES += $(LOCAL_DIR)/include
 
 MODULE_SRCS += \
 	$(LOCAL_DIR)/tests.c \