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>

arch/sandbox/Kconfig

@@ -35,4 +35,9 @@ config SANDBOX_64BIT
 
 endchoice
 
+config SANDBOX_BITS_PER_LONG
+	int
+	default 32 if SANDBOX_32BIT
+	default 64 if SANDBOX_64BIT
+
 endmenu

board/sandbox/README.sandbox

@@ -24,6 +24,9 @@ single board in board/sandbox.
 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 @@ Note:
       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:
 

scripts/config_whitelist.txt

@@ -1951,7 +1951,6 @@ CONFIG_SAMSUNG
 CONFIG_SAMSUNG_ONENAND
 CONFIG_SANDBOX_ARCH
 CONFIG_SANDBOX_BIG_ENDIAN
-CONFIG_SANDBOX_BITS_PER_LONG
 CONFIG_SANDBOX_SDL
 CONFIG_SANDBOX_SPI_MAX_BUS
 CONFIG_SANDBOX_SPI_MAX_CS