build: Apply OPT_FLAGS after platform Makefile.inc include

This allows the platform to specify another optimization level than
the default (like -Os if space is limited)

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

src/Makefile

@@ -8,12 +8,9 @@ MAKEFLAGS += --no-print-dir
 Q := @
 endif
 
-OPT_FLAGS ?= -O2
-
 CFLAGS += -Wall -Wextra -Werror -Wno-char-subscripts -Wno-cast-function-type \
-	$(OPT_FLAGS) -std=gnu99 -g3 -MD \
+	-std=gnu99 -g3 -MD \
 	-I. -Iinclude -Iplatforms/common -I$(PLATFORM_DIR)
-LDFLAGS += $(OPT_FLAGS)
 
 ifeq ($(ENABLE_DEBUG), 1)
 CFLAGS += -DENABLE_DEBUG
@@ -60,6 +57,10 @@ SRC =			\
 
 include $(PLATFORM_DIR)/Makefile.inc
 
+OPT_FLAGS ?= -O2
+CFLAGS += $(OPT_FLAGS)
+LDFLAGS += $(OPT_FLAGS)
+
 ifndef TARGET
 TARGET=blackmagic
 endif