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/common/stdio.c b/common/stdio.c
index d4acc52..4083e4e 100644
--- a/common/stdio.c
+++ b/common/stdio.c
@@ -336,7 +336,7 @@
 				       dev->name);
 		}
 	}
-#ifdef CONFIG_SYS_I2C
+#ifdef CONFIG_SYS_I2C_LEGACY
 	i2c_init_all();
 #endif
 	if (IS_ENABLED(CONFIG_DM_VIDEO)) {