sandbox: move source files from board/ to arch/sandbox/

Prior to commit 33a02da0, all boards must have board/${BOARD}/
or board/${VENDOR}/${BOARD}/ directory.
Now this rule is obsolete.

It looks weird that sandbox defines "vendor" and "board" just for
meeting the old U-Boot directory structure.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/lib/Makefile b/arch/sandbox/lib/Makefile
index 4c1a38d..6480ebf 100644
--- a/arch/sandbox/lib/Makefile
+++ b/arch/sandbox/lib/Makefile
@@ -8,4 +8,4 @@
 #
 
 
-obj-y	+= interrupts.o
+obj-y	+= interrupts.o sandbox.o