bma42x: add new BMA421/BMA425 driver

I wrote this for the PineTime, and all available sensors (accelerometer,
step counter, temperature sensor) do work.

This commit also includes two "configuration files", that actually
appear to be firmware files to run on the accelerometer for special
features like step counting.
I'm not sure where they originally came for, and I don't know the
copyright status of them. However, Bosch has open-sourced the BMA423
driver which includes a similar binary blob and the InfiniTime and
Wasp-OS projects have been shipping these blobs without issues, so I
think it's reasonably safe to include these binary blobs directly in the
source.

examples/bma42x/main.go

@@ -0,0 +1,54 @@
+package main
+
+// Smoke test for the BMA421/BMA425 sensors.
+// Warning: this code has _not been tested_. It's only here as a smoke test.
+
+import (
+	"fmt"
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers"
+	"tinygo.org/x/drivers/bma42x"
+)
+
+func main() {
+	time.Sleep(5 * time.Second)
+
+	i2cBus := machine.I2C1
+	i2cBus.Configure(machine.I2CConfig{
+		Frequency: 400 * machine.KHz,
+		SDA:       machine.SDA_PIN,
+		SCL:       machine.SCL_PIN,
+	})
+
+	sensor := bma42x.NewI2C(i2cBus, bma42x.Address)
+	err := sensor.Configure(bma42x.Config{
+		Device:   bma42x.DeviceBMA421 | bma42x.DeviceBMA425,
+		Features: bma42x.FeatureStepCounting,
+	})
+	if err != nil {
+		println("could not configure BMA421/BMA425:", err)
+		return
+	}
+
+	if !sensor.Connected() {
+		println("BMA42x not connected")
+		return
+	}
+
+	for {
+		time.Sleep(time.Second)
+
+		err := sensor.Update(drivers.Acceleration | drivers.Temperature)
+		if err != nil {
+			println("Error reading sensor", err)
+			continue
+		}
+
+		fmt.Printf("Temperature: %.2f °C\n", float32(sensor.Temperature())/1000)
+
+		accelX, accelY, accelZ := sensor.Acceleration()
+		fmt.Printf("Acceleration: %.2fg %.2fg %.2fg\n", float32(accelX)/1e6, float32(accelY)/1e6, float32(accelZ)/1e6)
+	}
+}