[build] Fix building on broken versions of GNU binutils

Some versions of GNU objcopy (observed with binutils 2.23.52.0.1 on
CentOS 7.0.1406) document the -D/--enable-deterministic-archives
option but fail to recognise the short form of the option.

Work around this problem by using the long form of the option.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
diff --git a/src/Makefile.housekeeping b/src/Makefile.housekeeping
index 9be7e7e..0deb15c 100644
--- a/src/Makefile.housekeeping
+++ b/src/Makefile.housekeeping
@@ -1167,7 +1167,8 @@
 	$(Q)$(RM) $(BLIB)
 	$(QM)$(ECHO) "  [AR] $@"
 	$(Q)$(AR) rD $@ $(sort $(BLIB_OBJS))
-	$(Q)$(OBJCOPY) -D --prefix-symbols=$(SYMBOL_PREFIX) $@
+	$(Q)$(OBJCOPY) --enable-deterministic-archives \
+		       --prefix-symbols=$(SYMBOL_PREFIX) $@
 	$(Q)$(RANLIB) -D $@
 blib : $(BLIB)