targets: add target circuitplay-bluefruit

Add a target for the Adafruit Circuit Playground Bluefruit, which is
based on the nRF52840. Adds the necessary code for the machine
package and the json and linker script files in the targets directory.
The machine package code is based on board_circuitplay_express.go,
with modifications made by consulting the wiring diagram on the
adafruit website here:
https://learn.adafruit.com/adafruit-circuit-playground-bluefruit/downloads

Also adds support to the uf2 conversion packacge to set the familyID
field. The Circuit Playground Bluefruit firmware rejects uf2 files
without the family id set to 0xADA52840 (and without the flag specifying
that the family id is present).

src/machine/board_circuitplay_bluefruit.go

@@ -0,0 +1,73 @@
+// +build circuitplay_bluefruit
+
+package machine
+
+const HasLowFrequencyCrystal = true
+
+// GPIO Pins
+const (
+	D0  = P0_30
+	D1  = P0_14
+	D2  = P0_05
+	D3  = P0_04
+	D4  = P1_02
+	D5  = P1_15
+	D6  = P0_02
+	D7  = P1_06
+	D8  = P0_13
+	D9  = P0_29
+	D10 = P0_03
+	D11 = P1_04
+	D12 = P0_26
+	D13 = P1_14
+)
+
+// Analog Pins
+const (
+	A1 = P0_02
+	A2 = P0_29
+	A3 = P0_03
+	A4 = P0_04
+	A5 = P0_05
+	A6 = P0_30
+	A7 = P0_14
+	A8 = P0_28
+	A9 = P0_31
+)
+
+const (
+	LED       = D13
+	NEOPIXELS = D8
+
+	BUTTONA = D4
+	BUTTONB = D5
+	SLIDER  = D7 // built-in slide switch
+
+	BUTTON  = BUTTONA
+	BUTTON1 = BUTTONB
+
+	LIGHTSENSOR = A8
+	TEMPSENSOR  = A9
+)
+
+// UART0 pins (logical UART1)
+const (
+	UART_TX_PIN = P0_14 // PORTB
+	UART_RX_PIN = P0_30 // PORTB
+)
+
+// I2C pins
+const (
+	SDA_PIN = P0_05 // I2C0 external
+	SCL_PIN = P0_04 // I2C0 external
+
+	SDA1_PIN = P0_00 // I2C1 internal
+	SCL1_PIN = P0_01 // I2C1 internal
+)
+
+// SPI pins (internal flash)
+const (
+	SPI0_SCK_PIN  = P0_19 // SCK
+	SPI0_MOSI_PIN = P0_21 // MOSI
+	SPI0_MISO_PIN = P0_23 // MISO
+)