sandbox: Add instructions about building on 32-bit machines

Sandbox is built with 64-bit ints by default. This doesn't work properly on
32-bit machines.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

board/sandbox/README.sandbox

@@ -44,6 +44,9 @@ 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:

include/configs/sandbox.h

@@ -27,7 +27,10 @@
 
 #define CONFIG_SYS_STDIO_DEREGISTER
 
-/* Number of bits in a C 'long' on this architecture */
+/*
+ * Number of bits in a C 'long' on this architecture. Set this to 32 when
+ * building on a 32-bit machine.
+ */
 #define CONFIG_SANDBOX_BITS_PER_LONG	64
 
 #define CONFIG_LMB