tools/mpremote/commands.py: Validate romfs image.

[iabdalkader]

Summary

Currently the tool allows writing invalid romfs image, with a bad header or images smaller than minimum size, and only checks the image extension.
This patch allows deploying romfs with any extension, and validates the image header and minimum size before writing.

Testing

Before this patch, I tested deploying 1 byte images, or all zero images, and in both cases the tool deploys them as long as they have the correct extension. Deeper validation could be performed in both the tool and runtime, as I've encountered a runtime issue with a corrupted image (during development) at least once.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (74a5bf9) to head (9e9be83).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #17037   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files         169      169           
  Lines       21890    21890           
=======================================
  Hits        21571    21571           
  Misses        319      319           

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     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