i2c: Rename CONFIG_SYS_I2C to CONFIG_SYS_I2C_LEGACY

It is quite confusing that CONFIG_SYS_I2C selects the legacy I2C and
CONFIG_DM_I2C selects the current I2C. The deadline to migrate I2C is less
than a year away.

Also we want to have a CONFIG_I2C for U-Boot proper just like we have
CONFIG_SPL_I2C for SPL, so we can simplify the Makefile rules.

Rename this symbol so it is clear it is going away.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/include/configs/sniper.h b/include/configs/sniper.h
index 4747e74..6ef96df 100644
--- a/include/configs/sniper.h
+++ b/include/configs/sniper.h
@@ -53,7 +53,7 @@
  * I2C
  */
 
-#define CONFIG_SYS_I2C
+#define CONFIG_SYS_I2C_LEGACY
 #define CONFIG_I2C_MULTI_BUS
 
 /*