allow SKIP_STRIP to be set in the env

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/Makefile.flags b/Makefile.flags
index 60bb888..363300b 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -4,7 +4,7 @@
 
 BB_VER = $(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
 export BB_VER
-SKIP_STRIP = n
+SKIP_STRIP ?= n
 
 # -std=gnu99 needed for [U]LLONG_MAX on some systems
 CPPFLAGS += $(call cc-option,-std=gnu99,)