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Now this rule is obsolete.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>

arch/sandbox/lib/Makefile

@@ -8,4 +8,4 @@
 #
 
 
-obj-y	+= interrupts.o
+obj-y	+= interrupts.o sandbox.o

board/sandbox/sandbox/Makefile

@@ -1,7 +0,0 @@
-#
-# Copyright (c) 2011 The Chromium OS Authors.
-#
-# SPDX-License-Identifier:	GPL-2.0+
-#
-
-obj-y	:= sandbox.o

boards.cfg

@@ -1179,7 +1179,7 @@ Active  powerpc     ppc4xx         -           xilinx          ppc405-generic
 Active  powerpc     ppc4xx         -           xilinx          ppc405-generic      xilinx-ppc405-generic_flash          xilinx-ppc405-generic:SYS_TEXT_BASE=0xF7F60000,RESET_VECTOR_ADDRESS=0xF7FFFFFC                                                    Ricardo Ribalda <ricardo.ribalda@uam.es>
 Active  powerpc     ppc4xx         -           xilinx          ppc440-generic      xilinx-ppc440-generic                xilinx-ppc440-generic:SYS_TEXT_BASE=0x04000000,RESET_VECTOR_ADDRESS=0x04100000,BOOT_FROM_XMD=1                                    Ricardo Ribalda <ricardo.ribalda@uam.es>
 Active  powerpc     ppc4xx         -           xilinx          ppc440-generic      xilinx-ppc440-generic_flash          xilinx-ppc440-generic:SYS_TEXT_BASE=0xF7F60000,RESET_VECTOR_ADDRESS=0xF7FFFFFC                                                    Ricardo Ribalda <ricardo.ribalda@uam.es>
-Active  sandbox     sandbox        -           sandbox         sandbox             sandbox                              -                                                                                                                                 Simon Glass <sjg@chromium.org>
+Active  sandbox     sandbox        -           -               <none>              sandbox                              -                                                                                                                                 Simon Glass <sjg@chromium.org>
 Active  sh          sh2            -           renesas         rsk7203             rsk7203                              -                                                                                                                                 Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>:Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
 Active  sh          sh2            -           renesas         rsk7264             rsk7264                              -                                                                                                                                 Phil Edworthy <phil.edworthy@renesas.com>
 Active  sh          sh2            -           renesas         rsk7269             rsk7269                              -                                                                                                                                 -