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>
diff --git a/arch/sandbox/include/asm/u-boot.h b/arch/sandbox/include/asm/u-boot.h
index 8279894..ddcd6fb 100644
--- a/arch/sandbox/include/asm/u-boot.h
+++ b/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