nrf52840: improve USBCDC

[sago35]

This PR improves the USBCDC speed of nrf52840.
The atsamd5x version is #1481 .
The atsamd2x version is #1636 .

• 13 byte : fmt.Printf("hello world\r\n")
  • old : 631 us
  • new : 64 us
• 11 byte : println("hello world")
  • old : 320 us
  • new : 15 us
• 1024 byte + "\r\n"
  • old : 28.746 ms
  • new : 1.249 ms
• 1024 * 10 byte + "\r\n"
  • old : 515.017 ms
  • new : 12.746 ms

windows 10 + feather-nrf52840

[sago35]

Even in the unmodified version, it often crashes immediately after tinygo-flash.
When it crashes, I sometimes get the following message

panic: runtime error: goroutine stack overflow

After resetting several times, the problem will no longer occur.

[deadprogram]

Once thing I have noticed is that you really need to update nrf52840 bootloader to the latest version from Adafruit (0.4.0) to avoid issues with flashing. See tinygo-org/tinygo-site#143 for info on how to do this.

[sago35]

Updated to 0.4.1.
When jlink is not used, there seems to be no problem.
After writing from jlink, it looks like I need to perform a reset several times myself.

[deadprogram]

I have not tried to use jlink on this board, but that does not entirely surprise me. Might need an issue reported upstream to the bootloader repo.

[deadprogram]

Absolutely works better/faster @sago35 thanks for working on this! Now merging.

[deadprogram]

I meant to squash this first, but was too quick on the click. Sorry about that.