Add bootloader stub generator for ESP32-C3

[duncandrennan]

@rojer

I managed to get the stub code for the ESP32-C3 to compile and generate a stub. I have no idea whether this will work - it will definitely need a review.

• I think the linker script is okay, but I would like a second set of eyes on this.
• I've hacked wrap_stub.py to accept the machine type of the generated ELF file
• Use duncandrennan/mongoose-os to build the ESP-IDF 4.3-dev docker container

[duncandrennan]

Noting that https://github.com/torvalds/linux/blob/master/include/uapi/linux/elf-em.h lists

#define EM_RISCV 243 /* RISC-V */

so the 0xf3 is correct in wrap_stub.py

[rojer]

looks reasonable overall but you should make sure it works. i suggest using mos flash-read and mos flash-write to read and write flash, while we cannot yet generate working firmware.

[duncandrennan]

I'll work towards that as the next step.

[rojer]

ok, cool