Robust packet reading

[dhalbert]

• Read characters until a start-of-packet character is read.
• Fix incorrect packet length for LocationPacket.

This fixes some but not all issues with adafruit/circuitpython#1568.

[dhalbert]

More comprehensive test program:

from adafruit_ble.uart import UARTServer
from adafruit_bluefruit_connect.packet import Packet
from adafruit_bluefruit_connect.accelerometer_packet import AccelerometerPacket
from adafruit_bluefruit_connect.button_packet import ButtonPacket
from adafruit_bluefruit_connect.color_packet import ColorPacket
from adafruit_bluefruit_connect.gyro_packet import GyroPacket
from adafruit_bluefruit_connect.location_packet import LocationPacket
from adafruit_bluefruit_connect.magnetometer_packet import MagnetometerPacket
from adafruit_bluefruit_connect.quaternion_packet import QuaternionPacket


uart_server = UARTServer(buffer_size=256)

while True:
    uart_server.start_advertising()
    while not uart_server.connected:
        pass

    # Now we're connected

    while uart_server.connected:
        if uart_server.in_waiting:
            packet = Packet.from_stream(uart_server)
            if isinstance(packet, AccelerometerPacket):
                print("Acceleration:", packet.x, packet.y, packet.z)
            elif isinstance(packet, ButtonPacket):
                print("Button", packet.button, packet.pressed)
            elif isinstance(packet, ColorPacket):
                print("Color:", packet.color)
            elif isinstance(packet, GyroPacket):
                print("Gyro:", packet.x, packet.y, packet.z)
            elif isinstance(packet, LocationPacket):
                print("Location:", packet.latitude, packet.longitude, packet.altitude)
            elif isinstance(packet, MagnetometerPacket):
                print("Magnetometer:", packet.x, packet.y, packet.z)
            elif isinstance(packet, QuaternionPacket):
                print("Quaternion:", packet.x, packet.y, packet.z, packet.w)

    # If we got here, we lost the connection. Go up to the top and start
    # advertising again and waiting for a connection.

[kattni]

This is great! Location is working now, and I was able to run the updated demo for 20 minutes. At that point it stopped, however, I was able to interact with the serial console/REPL still, so it didn't crash the board, the phone simply stopped sending data. Thank you for the update!

Travis is unhappy. I'll merge it once that's sorted!