build system: add PIE build option

Makefile

@@ -315,6 +315,8 @@ AFLAGS_KERNEL	=
 # Use LINUXINCLUDE when you must reference the include/ directory.
 # Needed to be compatible with the O= option
 CFLAGS		:= $(CFLAGS)
+# Added only to final link stage of busybox binary
+CFLAGS_busybox	:= $(CFLAGS_busybox)
 CPPFLAGS	:= $(CPPFLAGS)
 AFLAGS		:= $(AFLAGS)
 LDFLAGS		:= $(LDFLAGS)
@@ -580,7 +582,7 @@ quiet_cmd_busybox__ ?= LINK    $@
       cmd_busybox__ ?= $(srctree)/scripts/trylink \
       "$@" \
       "$(CC)" \
-      "$(CFLAGS)" \
+      "$(CFLAGS) $(CFLAGS_busybox)" \
       "$(LDFLAGS) $(EXTRA_LDFLAGS)" \
       "$(core-y)" \
       "$(libs-y)" \