sandbox: Move CONFIG_SYS_I2C_SANDBOX to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher<hs@denx.de>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index efc020f..56e7deb 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -19,3 +19,4 @@
 CONFIG_BOOTSTAGE_REPORT=y
 CONFIG_SANDBOX_GPIO=y
 CONFIG_SYS_VSNPRINTF=y
+CONFIG_SYS_I2C_SANDBOX=y