[gen3] make sure that extended system-part1 is compatible with older bootloaders as well as 6.1.2

[avtolstoy]

Description

In order to allow older bootloaders to boot into a larger system-part1 without any modifications we can keep a “fake” module info header in the normal/old location @ 0x30000 + 0x200 (vector table size). All it needs to contain is proper platform id, start and end addresses, so that (end - start) equals sizeof(vector table) + sizeof(module info) + sizeof(dynalib table). This whole section needs to be CRC32-d and CRC32 appended immediately after dynalib. This should allow the checks in https://github.com/particle-iot/device-os/blob/v5.9.0/bootloader/src/nRF52840/dfu_usb.c#L15 to pass and current bootloaders will correctly jump into a larger system-part1. All at the cost of 24 bytes of module info header and a kilo of makefile/linker magic.

PR also enables running 5.8.0 to 6.0.0 applications now that everything has been merged together.

Steps to Test

1. Flash a device with 5.9.0 (including bootloader)
2. Flash system-part1 out of this branch over DFU
3. The device should boot correctly, SMH, reset and still boot correctly

References

https://www.notion.so/Gen-3-platforms-out-of-flash-space-for-Device-OS-features-13ec13233b404d739ae947b6350d4332?d=bdc4ff20fa9e4287b2b1295616d085c0&pvs=4#2e5cf972e1a7410c82c1365b7277eb7a

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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)