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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index b911c02..efc020f 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -18,3 +18,4 @@
 CONFIG_BOOTSTAGE=y
 CONFIG_BOOTSTAGE_REPORT=y
 CONFIG_SANDBOX_GPIO=y
+CONFIG_SYS_VSNPRINTF=y