sandbox: Make u-boot-sandbox.h a private header

Rather than including this arch-specific header file in common.h, include
it from within sandbox's u-boot.h header.

Also drop the comment about something to be fixed.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/include/asm/u-boot.h

@@ -22,6 +22,7 @@
 
 /* Use the generic board which requires a unified bd_info */
 #include <asm-generic/u-boot.h>
+#include <asm/u-boot-sandbox.h>
 
 /* For image.h:image_check_target_arch() */
 #define IH_ARCH_DEFAULT IH_ARCH_SANDBOX

include/common.h

@@ -434,7 +434,7 @@ static inline int setenv_addr(const char *varname, const void *addr)
 # include <asm/u-boot-x86.h>
 #endif /* CONFIG_X86 */
 #ifdef CONFIG_SANDBOX
-# include <asm/u-boot-sandbox.h>	/* TODO(sjg) what needs to be fixed? */
+# include <asm/u-boot.h>
 #endif
 #ifdef CONFIG_NDS32
 # include <asm/mach-types.h>