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/README b/README
index 63ac8d3..4fdc49f 100644
--- a/README
+++ b/README
@@ -1461,9 +1461,12 @@
 		In such cases CONFIG_GPIO_LED_INVERTED_TABLE may be defined
 		with a list of GPIO LEDs that have inverted polarity.
 
-- I2C Support:	CONFIG_SYS_I2C
+- I2C Support:	CONFIG_SYS_I2C_LEGACY
 
-		This enable the NEW i2c subsystem, and will allow you to use
+		Note: This is deprecated in favour of driver model. Use
+		CONFIG_DM_I2C instead.
+
+		This enable the legacy i2c subsystem, and will allow you to use
 		i2c commands at the u-boot command line (as long as you set
 		    CONFIG_SYS_I2C_SOFT_SPEED and CONFIG_SYS_I2C_SOFT_SLAVE
 		    for defining speed and slave address