i2c: add stubs for targets without i2c

src/runtime/src/i2c.rs

@@ -1,5 +1,7 @@
 use libboard_zynq;
 
+#[cfg(feature = "target_zc706")]
+mod i2c {
     use crate::artiq_raise;
 
     static mut I2C_BUS: Option<libboard_zynq::i2c::I2c> = None;
@@ -66,3 +68,34 @@ pub fn init() {
         i2c.init().expect("I2C bus initialization failed");
         unsafe { I2C_BUS = Some(i2c) };
     }
+}
+
+#[cfg(not(feature = "target_zc706"))]
+mod i2c {
+    use crate::artiq_raise;
+
+    pub extern fn start(_busno: i32) {
+        artiq_raise!("I2CError", "No I2C bus");
+    }
+
+    pub extern fn restart(_busno: i32) {
+        artiq_raise!("I2CError", "No I2C bus");
+    }
+
+    pub extern fn stop(_busno: i32) {
+        artiq_raise!("I2CError", "No I2C bus");
+    }
+
+    pub extern fn write(_busno: i32, _data: i32) -> bool {
+        artiq_raise!("I2CError", "No I2C bus");
+    }
+
+    pub extern fn read(_busno: i32, _ack: bool) -> i32 {
+        artiq_raise!("I2CError", "No I2C bus");
+    }
+
+    pub fn init() {
+    }
+}
+
+pub use i2c::*;