[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

target/stm32-h103/rules.mk

@@ -10,7 +10,7 @@ DEFINES += \
 	ENABLE_UART1=1 \
 	TARGET_HAS_DEBUG_LED=1
 
-INCLUDES += -I$(LOCAL_DIR)/include
+GLOBAL_INCLUDES += $(LOCAL_DIR)/include
 
 MODULE_SRCS += \
 	$(LOCAL_DIR)/init.c