balena-bootloader.bbclass: Move common bits from devices to class

[mtoman]

After adding the balena bootloader to a handful of devices, some stuff keeps getting copy/pasted into the bootloader recipes.

This patch moves the common bits to balena-bootloader.bbclass to avoid duplication and make it easier to use.

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Contributor checklist

• Changes have been tested
  • Covered in automated test suite
  • Manual test case recorded
• Change-type present on at least one commit
• Signed-off-by is present
• The PR complies with the Open Embedded Commit Patch Message Guidelines

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