[FEATURE] raspberrypi 2/3/4/5 firmware flashing automation with make flash (no uSD).

### Is your feature request related to a problem? Please describe.

* Right now we need to build NuttX, then mount uSD card, copy `nuttx.bin` and some other files to the uSD card, umount, and move card from the build host to the target board slot.
* Testing requires manual interaction that blocks automated testing.

### Describe the solution you'd like

* It would be great to have `make flash` that would remotely flash and boot the NuttX on rPI 2/3/4/5 boards :-)
* Because additional the USB-UART is required for now for `nsh` console maybe `xmodem` can be used to provide `nuttx.bin` over U-Boot?
* I know @linguini1 works on the NuttX port to rPI 4B and I bought some 1/2/8GB RAM boards for testing. We can start with rPI 4B :-)

### Describe alternatives you've considered

* @lupyuen found SDWire interface [1] to emulate uSD card and enable testing automation. But this is dedicated hardware and for many boards comes quite expensive more expensive than the boards themselves :-P

[1] https://shop.3mdeb.com/product/sdwire/

### Verification

- [x] I have verified before submitting the report.