flashstub: Fix ELF to hexdump conversion

[dsdve]

Detailed description

Meson is currently set up to build hexdumps of entire ELF files instead of just the relevant part of the binary (which is the .text section in all cases).

This adds an intermediate objcopy to extract the section in binary format and directs that to the hexdump step to produce the correct output.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

None.

[dragonmux]

Got only the one review note and with that addressed, we'll be happy to merge this.