Writing to the UART takes time and that may not be a good idea inside an
interrupt, but it is essential for debugging sometimes (especially since
USB-CDC typically doesn't work inside an interrupt).
This fixes UART support in interrupts for the RP2040 at least. You can
test it with `-serial=uart` and connecting a USB-UART adapter to the
right pins.
Don't block forever if there's nothing to receive. On the other hand,
process the entire FIFO, not just a single byte.
This fixes an issue where the rp2350 would hang after programming through
openocd, where the UART0 interrupt would be spuriously pending.
Ayke van Laethem
Elias Naur
Patricio Whittingslow