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/asm-generic/global_data.h b/include/asm-generic/global_data.h
index 5fed4db..e550703 100644
--- a/include/asm-generic/global_data.h
+++ b/include/asm-generic/global_data.h
@@ -277,7 +277,7 @@
 	 */
 	void *trace_buff;
 #endif
-#if defined(CONFIG_SYS_I2C)
+#if defined(CONFIG_SYS_I2C_LEGACY)
 	/**
 	 * @cur_i2c_bus: currently used I2C bus
 	 */