sandbox: convert makefiles to Kbuild style

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/lib/Makefile b/arch/sandbox/lib/Makefile
index 993fb4e..4c1a38d 100644
--- a/arch/sandbox/lib/Makefile
+++ b/arch/sandbox/lib/Makefile
@@ -7,28 +7,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
 
-LIB	= $(obj)lib$(ARCH).o
-
-COBJS-y	+= interrupts.o
-
-SRCS	:= $(COBJS-y:.o=.c)
-OBJS	:= $(addprefix $(obj),$(COBJS-y))
-
-# Always build libsandbox.o
-TARGETS	:= $(LIB)
-
-all:	$(TARGETS)
-
-$(LIB):	$(obj).depend $(OBJS)
-	$(call cmd_link_o_target, $(OBJS))
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
-
-#########################################################################
+obj-y	+= interrupts.o