sandbox: Move CONFIG_SYS_VSNPRINTF to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b44f709..3f1401a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -7,6 +7,9 @@
 config USE_PRIVATE_LIBGCC
 	default y
 
+config SYS_VSNPRINTF
+	default y
+
 choice
 	prompt "Target select"