Make examples friendlier for non GCC compilers and fixed slave_mem_i2c.c to compile on boards without I2C pins (#336)

spi/mpu9250_spi/mpu9250_spi.c

@@ -150,6 +150,4 @@ int main() {
 
         sleep_ms(100);
     }
-
-    return 0;
 }