Remove Nrf52 bootloader target and hex

[hathach]

To prevent adding hex file each time we modify bootloader. Bootloader comes with pre-builtin hex for S140 6.1.0 and S132 6.1.0 for feather & pca10056. Bootloader related targets and hex are removed, user need to clone the bootloader repo should they need to flash or upgrade board's bootloader.

• SD version for S140 is upgraded to 6.1.0, since it is binary compatible with 6.0.0, I can still work. However bootloader 6.1.0 has some bug fix, it is better to flash the new one.
• nrf52832 is left as it is, waiting for @arturo182
  - bootloader does also include nrfx and tinyusb, kind of duplication. But it is also standalone, have no idea to go around this.

[tannewt]

I don't think we should add any knowledge of the bootloader to CircuitPython. It's only used to provide convenience targets in the Makefile to flash the bootloader. Those targets should be removed. The documentation and Makefile in it's own repo is enough.

[tannewt]

This shouldn't be merged. The bootloader targets in the Makefile should be removed instead.

[hathach]

@tannewt the bootloader target in the makefile is to flash the bootloader on the device with jlink, or you are referring to other targets. Could you say the target name specifically.

[hathach]

Hmm, probably it is a bit confusing, I will update the readme a bit.

[hathach]

update: the readme does mentions dfu-flash and dfu-flash already, @tannewt would you mind telling which lines of the makefile you want to remove.

[hathach]

@dhalbert @tannewt maybe we shouldn't add bootloader as submodule at all. A download script like SD's one should be enough ?

[dhalbert]

I think @tannewt is saying to not have anything about the bootloader in the circuitpython repo. If they are not already there, you can move the Makefile targets that load the bootloader to the https://github.com/adafruit/Adafruit_nRF52_Bootloader repo. If someone wants to load the bootloader they can follow instructions in the bootloader repo and/or use Makefile targets there.

We don't have targets that load the UF2 bootloader in the atmel-samd/Makefile -- all the bootloader info is in the UF2 bootloader repo.

[hathach]

@dhalbert ah that make sense, thanks. I understand what @tannewt means now. Since bootloader target existed previously, I haven't done anything with it in this PR. I will remove bootloader and dfu-bootloader target.

[hathach]

@dhalbert maybe we should remove bootloader as the submodule as well, just tell people to clone the bootloader and flash it there in the readme.md

[hathach]

close this one by now, I need to do a few more things with bootloader repo and submit a separate PR.