Include bootloader update downloads

[deshipu]

Now that we have some bootloader updates, it would be nice to be able to point people to circuitpython.org to download them as well.

[dhalbert]

This is getting more important due to the need to update the bootloaders because of the MacOS 10.14.4 MakeCode issue: That MacOS version cannot see the BOOT drive when there's a MakeCode program loaded

Note that the board names for the bootloader and CircuitPython are not the same, and there's not even a one-to-one correspondence: some bootloaders are used for multiple similar boards.

[dhalbert]

@Racer1Tn Please post in the support forums https://support.adafruit.com so we can help you there. This is for circuitpython.org issues.

[dhalbert]

Fixed by #446.