sandbox: Convert SANDBOX_BITS_PER_LONG to Kconfig

Convert SANDBOX_BITS_PER_LONG to Kconfig and assign it a correct
number depending on which host we are going to build and run.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index 9dc2eb0..2e2c819 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -24,6 +24,9 @@
 CONFIG_SANDBOX_BIG_ENDIAN should be defined when running on big-endian
 machines.
 
+By default sandbox builds and runs on 64-bit hosts. If you are going to build
+and run sandbox on a 32-bit host, select CONFIG_SANDBOX_32BIT.
+
 Note that standalone/API support is not available at present.
 
 
@@ -44,10 +47,6 @@
       make sandbox_defconfig all NO_SDL=1
       ./u-boot
 
-   If you are building on a 32-bit machine you may get errors from __ffs.h
-   about shifting more than the machine word size. Edit the config file
-   include/configs/sandbox.h and change CONFIG_SANDBOX_BITS_PER_LONG to 32.
-
 U-Boot will start on your computer, showing a sandbox emulation of the serial
 console: