Cleanup 3rd party samples; update README.md; add some missing copyright, fix builds for boards without certain pin definitions

i2c/mcp9808_i2c/mcp9808_i2c.c

@@ -66,7 +66,7 @@ float mcp9808_convert_temp(uint8_t upper_byte, uint8_t lower_byte) {
     return temperature;
 }
 
-
+#ifdef i2c_default
 void mcp9808_set_limits() {
 
     //Set an upper limit of 30°C for the temperature
@@ -97,7 +97,7 @@ void mcp9808_set_limits() {
     buf[2] = crit_temp_lsb;;
     i2c_write_blocking(i2c_default, ADDRESS, buf, 3, false);
 }
-
+#endif
 
 int main() {
 
@@ -117,7 +117,6 @@ int main() {
     gpio_pull_up(PICO_DEFAULT_I2C_SCL_PIN);
     // Make the I2C pins available to picotool
     bi_decl(bi_2pins_with_func(PICO_DEFAULT_I2C_SDA_PIN, PICO_DEFAULT_I2C_SCL_PIN, GPIO_FUNC_I2C));
-#endif
 
     mcp9808_set_limits();
 
@@ -144,4 +143,5 @@ int main() {
 
         sleep_ms(1000);
     }
+#endif
 }