DAPLink BootLoader v243 will not allow JLink to be installed on a Micro:Bit

Hi!

I'm trying to get JLink on a Micro:Bit. It looks like I'm running:

```
# DAPLink Firmware - see https://mbed.com/daplink
Unique ID: 0000000050494e4500622003000000160000000097969901
HIC ID: 97969901
Auto Reset: 0
Automation allowed: 1
Overflow detection: 0
Daplink Mode: Bootloader
Bootloader Version: 0243
Interface Version: 0253
Git SHA: b403a07e3696cee1e116d44cbdd64446e056ce38
Local Mods: 0
USB Interfaces: MSD
Bootloader CRC: 0x32eb3cfd
Interface CRC: 0x53375800
Remount count: 0
```

When I try and flash JLink onto the device I get the following failures:

FAIL.TXT
```
An internal error has occurred
```

ASSERT.TXT
```
Assert
File: (file)
Line: 206
Source: Bootloader
```

Post-JLink Failed Flash DETAILS.TXT
```
# DAPLink Firmware - see https://mbed.com/daplink
Unique ID: 0000000050494e4500622003000000160000000097969901
HIC ID: 97969901
Auto Reset: 0
Automation allowed: 1
Overflow detection: 0
Daplink Mode: Bootloader
Bootloader Version: 0243
Git SHA: b403a07e3696cee1e116d44cbdd64446e056ce38
Local Mods: 0
USB Interfaces: MSD
Bootloader CRC: 0x32eb3cfd
Interface CRC: 0xd87a6179
Remount count: 0
```

Based on the error it seems like something with Bootloader v243.

I asked a friend who I know had an older Micro:Bit. He confirmed his bootloader version:
```
Version: 0234
Build: Oct 12 2015 14:39:34
```

We Flashed JLink and he got this message, which I assume means it worked:

```
This J-Link VMSD config can be used to flash a NordicSemi nRF51822 target device.
```

I cannot find any resources on:

1. Debugging this Bootloader error any further. (I'm also an embedded newb)
1. If it's possible to revert Bootloader versions in DAPLink.

If anyone could help with either of those, I'd really appreciate it. It's that _really close_ feeling! 💃

Start by locating the DAPLink bootloader assertion reported at line 206 and reproduce the JLink flashing failure with Bootloader v243. Compare the reported v243 and v234 behavior, then document whether JLink flashing is supported and whether a safe bootloader rollback path exists.

Written by the indexing model from the issue text.