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/phy62xx/phy6222/scripts/Make.defs

@@ -47,7 +47,7 @@ CXXPICFLAGS = $(ARCHPICFLAGS) $(CXXFLAGS)
 CPPFLAGS := $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS)
 AFLAGS := $(CFLAGS) -D__ASSEMBLY__
 
-CFLAGS += -ffunction-sections -fdata-sections
+CFLAGS +=
 
 NXFLATLDFLAGS1 = -r -Wl,-d -Wl,-warn-common
 NXFLATLDFLAGS2 = $(NXFLATLDFLAGS1) -T$(TOPDIR)/binfmt/libnxflat/gnu-nxflat-pcrel.ld -Wl,-no-check-sections
@@ -56,5 +56,3 @@ LDNXFLATFLAGS = -e main -s 2048
 ifeq ($(CONFIG_DEBUG_SYMBOLS),y)
   LDFLAGS += -g
 endif
-
-LDFLAGS += --gc-sections