compile/opt: add config DEBUG_OPT_UNUSED_SECTIONS

Enable this option to optimization the unused input sections with the
linker by compiling with " -ffunction-sections -fdata-sections ", and
linking with " --gc-sections ".

Signed-off-by: chao.an <anchao@xiaomi.com>

boards/arm/imx6/sabre-6quad/scripts/Make.defs

@@ -34,7 +34,7 @@ ifneq ($(CONFIG_DEBUG_NOOPT),y)
   ARCHOPTIMIZATION += $(MAXOPTIMIZATION) -fno-strict-aliasing
 endif
 
-ARCHCFLAGS += -fno-common -ffunction-sections -fdata-sections
+ARCHCFLAGS += -fno-common
 ARCHCXXFLAGS += -fno-common
 ARCHWARNINGS = -Wall -Wstrict-prototypes -Wshadow -Wundef
 ARCHWARNINGSXX = -Wall -Wshadow -Wundef
@@ -61,8 +61,6 @@ CXXELFFLAGS = $(CXXFLAGS) -mlong-calls # --target1-abs
 LDELFFLAGS = -r -e main
 LDELFFLAGS += -T $(call CONVERT_PATH,$(BOARD_DIR)$(DELIM)scripts$(DELIM)gnu-elf.ld)
 
-LDFLAGS += --gc-sections
-
 ifeq ($(CONFIG_DEBUG_SYMBOLS),y)
   LDFLAGS += -g
 endif