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/cmd/eeprom.c b/cmd/eeprom.c
index b3fd37c..efd6f3a 100644
--- a/cmd/eeprom.c
+++ b/cmd/eeprom.c
@@ -75,7 +75,7 @@
 	/* I2C EEPROM */
 #if CONFIG_IS_ENABLED(DM_I2C)
 	eeprom_i2c_bus = bus;
-#elif defined(CONFIG_SYS_I2C)
+#elif defined(CONFIG_SYS_I2C_LEGACY)
 	if (bus >= 0)
 		i2c_set_bus_num(bus);
 	i2c_init(CONFIG_SYS_I2C_SPEED, CONFIG_SYS_I2C_SLAVE);