[build] add boards specific flags when needed

and activate LTO compilation to reduce code size on small board naze32

conf/Makefile.sim

@@ -50,14 +50,15 @@ INCLUDES += -I $(shell $(OCAMLC) -where)
 CFLAGS  = -W -Wall
 CFLAGS += $(INCLUDES)
 CFLAGS += $($(TARGET).CFLAGS)
-CFLAGS += $(USER_CFLAGS)
+CFLAGS += $(USER_CFLAGS) $(BOARD_CFLAGS)
 CFLAGS += -fPIC
 CFLAGS += -O$(OPT)
 CFLAGS += -g
 CFLAGS += -std=gnu99
 CFLAGS += $(shell pkg-config --cflags-only-I ivy-glib)
 
-LDFLAGS		=	-lm
+LDFLAGS	 =	-lm
+LDFLAGS += $(BOARD_LDFLAGS)
 
 LIBFLAGS = -shared