Make requiring I2C pullups be optional

ports/atmel-samd/common-hal/busio/I2C.c

@@ -76,6 +76,7 @@ void common_hal_busio_i2c_construct(busio_i2c_obj_t *self,
         mp_raise_ValueError(translate("Invalid pins"));
     }
 
+#if CIRCUITPY_REQUIRE_I2C_PULLUPS
     // Test that the pins are in a high state. (Hopefully indicating they are pulled up.)
     gpio_set_pin_function(sda->number, GPIO_PIN_FUNCTION_OFF);
     gpio_set_pin_function(scl->number, GPIO_PIN_FUNCTION_OFF);
@@ -98,6 +99,8 @@ void common_hal_busio_i2c_construct(busio_i2c_obj_t *self,
         reset_pin_number(scl->number);
         mp_raise_RuntimeError(translate("SDA or SCL needs a pull up"));
     }
+#endif
+
     gpio_set_pin_function(sda->number, sda_pinmux);
     gpio_set_pin_function(scl->number, scl_pinmux);
 

ports/nrf/common-hal/busio/I2C.c

@@ -115,6 +115,7 @@ void common_hal_busio_i2c_construct(busio_i2c_obj_t *self, const mcu_pin_obj_t *
         mp_raise_ValueError(translate("All I2C peripherals are in use"));
     }
 
+#if CIRCUITPY_REQUIRE_I2C_PULLUPS
     // Test that the pins are in a high state. (Hopefully indicating they are pulled up.)
     nrf_gpio_cfg_input(scl->number, NRF_GPIO_PIN_PULLDOWN);
     nrf_gpio_cfg_input(sda->number, NRF_GPIO_PIN_PULLDOWN);
@@ -132,6 +133,7 @@ void common_hal_busio_i2c_construct(busio_i2c_obj_t *self, const mcu_pin_obj_t *
         reset_pin_number(scl->number);
         mp_raise_RuntimeError(translate("SDA or SCL needs a pull up"));
     }
+#endif
 
     nrfx_twim_config_t config = NRFX_TWIM_DEFAULT_CONFIG(scl->number, sda->number);
 

py/circuitpy_mpconfig.mk

@@ -310,6 +310,13 @@ CIRCUITPY_BITBANG_APA102 = 0
 endif
 CFLAGS += -DCIRCUITPY_BITBANG_APA102=$(CIRCUITPY_BITBANG_APA102)
 
+# Should busio.I2C() check for pullups?
+# Some boards in combination with certain peripherals may not want this.
+ifndef CIRCUITPY_REQUIRE_I2C_PULLUPS
+CIRCUITPY_REQUIRE_I2C_PULLUPS = 1
+endif
+CFLAGS += -DCIRCUITPY_REQUIRE_I2C_PULLUPS=$(CIRCUITPY_REQUIRE_I2C_PULLUPS)
+
 # REPL over BLE
 ifndef CIRCUITPY_SERIAL_BLE
 CIRCUITPY_SERIAL_BLE = 0