Ignore defaults.hjson values if already set

[zvecr]

Description

{
    "development_board": "blackpill_f401",
    "bootloader": "tinyuf2"
}

Previously, this would have resulted in bootloader being overwritten to stm-dfu.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[sigprof]

This actually fixes another instance of the problem noted in #19195 (comment) (another part of that problem was a wrong "pin_compatible": "promicro" setting in the definition of "development_board": "elite_c", but without this fix the "pin_compatible": "elite_c" override in info.json did not work, therefore a rules.mk override needed to be added).

With both #19511 and #19512 in the argo_works/ishi/80/mk0_avr_extra code compiles with CONVERT_TO=elite_pi even when both PIN_COMPATIBLE = elite_c and "pin_compatible": "elite_c" are removed (just "development_board": "elite_c" is enough).