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/drivers/i2c/Makefile b/drivers/i2c/Makefile
index 06a1150..c2eb24e 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -11,7 +11,7 @@
 obj-$(CONFIG_$(SPL_)I2C_CROS_EC_LDO) += cros_ec_ldo.o
 
 obj-$(CONFIG_I2C_MV) += mv_i2c.o
-obj-$(CONFIG_SYS_I2C) += i2c_core.o
+obj-$(CONFIG_SYS_I2C_LEGACY) += i2c_core.o
 obj-$(CONFIG_SYS_I2C_ASPEED) += ast_i2c.o
 obj-$(CONFIG_SYS_I2C_AT91) += at91_i2c.o
 obj-$(CONFIG_SYS_I2C_CADENCE) += i2c-cdns.o