Debounce BOOT0 readout in the hot loop (#2633)

Debounce BOOT0 readout in the hot loop (#2633)

Connecting a USB cable sometimes introduces noise on the pins of the USB
to UART converter leading to a low BOOT0 pin. In the main loop of TX
this is checked for indication of a backpack update. Depending on timing
this leads to the whole TX hanging since the serial connection is closed
and the device enters passthrough mode.

Introduce a simple software debouncing to ensure the state of the pin
has settled before entering the update mode.

Co-authored-by: Johannes Hutter <johannes.hutter@helsing.ai>