sandbox: Add main program

Add a main program so that we can run U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/Makefile b/arch/sandbox/cpu/Makefile
index 1a0b260..e5e860b 100644
--- a/arch/sandbox/cpu/Makefile
+++ b/arch/sandbox/cpu/Makefile
@@ -30,7 +30,7 @@
 
 LIB	= $(obj)lib$(CPU).o
 
-COBJS	:= cpu.o os.o
+COBJS	:= cpu.o start.o os.o
 
 SRCS	:= $(COBJS:.o=.c)
 OBJS	:= $(addprefix $(obj),$(COBJS))