[rtl872x] Allow KM4 SDK Bootloader images to boot

[scott-brust]

Problem

We want to allow Particle built prebootloader binaries to boot Realtek SDK based KM4 bootloader binaries. This is broken in the current Device OS.

Solution

The RTK SDK KM4 based bootloaders expect a FLASH_InitTypeDef flash_init_para structure to be present in BKUP_REG7 at boot. We had this originally, but since our bootloader does not need this, it was removed.
It doesnt appear that anything internal to DVOS uses BKUP_REG7, so this change should be safe

Steps to Test

1. Build prebootloader-part1 from this branch, flash to p2
2. Flash SDK KM4 Bootloader + APP images
3. Verify the KM4 bootloader + KM4 APP runs

Example App

Ask scott for KM4 SDK Bootloader binary

References

Links to the Community, Docs, Other Issues, etc..

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)