The `SWJ_CFG` field of the `AFIO_MAPR` register is write only and values
read are undefined (random). Hence, using `AFIO->MAPR |= mask;` to
enable flags can corrupt the state of the `SWJ_CFG` (configure it to
an unintended value).
Two helper functions have been introduced:
- `afio_mapr_read()` reads the value, but sanitizes the `SWJ_CFG` field
to zero
- `afio_mapr_write()` writes the given value, but applies the `SWJ_CFG`
configured by the board before writing.
Finally, the `nucleo-f103rb` and `bluepill*`/`blackpill*` boards have
been updated to no longer specify `STM32F1_DISABLE_JTAG`, as this
is handled by the `SWJ_CFG` setting (which defaults to disabling JTAG).
The STM32 line of microcontrollers comes with a bootloader in the ROM.
It provides the option to flash the device firmware in DFU mode (USB)
or via UART or SPI.
To enter the bootloader we have to jump to a specific address in memory,
but before reset the CPU to make sure the system is in a known state.
This enables us to use the usb_board_reset module on all STM32 platforms.
