ports/unix,windows: Add embedded romfs support with auto-mount.

[andrewleech]

Summary

Adds support for an embedded romfs image linked into the unix/windows binary. With ROMFS_IMG=path/to/romfs.img the image is embedded as a .rodata section via objcopy and registered as a read-only filesystem at startup. Auto-mount at /rom and (when present) prepend to sys.path. If the romfs contains main.py or main.mpy, it is executed in preference to the file-system search.

Also adds a make romfs ROMFS_DIR=... target on both ports for building images via mpremote, and MICROPY_ENABLE_COMPILER=0 guards in unix main.c so the port can be built compiler-less when only frozen .mpy modules are needed.

Testing

Unix build with and without ROMFS_IMG; Windows mingw build with embedded romfs; auto-execution of main.mpy from /rom.

[github-actions]

Code size report:

Reference:  extmod/moductypes: Be more defensive with uctypes_struct_agg_size args. [8a56be6]
Comparison: ports/unix,windows: Add embedded romfs support with auto-mount. [merge of af886f8]
  mpy-cross:    +0 +0.000% 
   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64: +1912 +0.223% standard[incl +48(data) +32(bss)]
      stm32:    +0 +0.000% PYBV10
      esp32:    +0 +0.000% ESP32_GENERIC
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32