Balena bootloader support for the raspberrypicm4-ioboard-sb device

[alexgg]

@resin-jenkins retest this please

[alexgg]

Passed:

• test-raspberrypi3-64
• test-raspberrypi4-64
• test-raspberrypi4-unipi-neuron
• test-rt-rpi-300
• test-raspberrypi3

Failed:

• test-revpi-connect
• test-raspberrypi0-2w-64
• test-raspberrypi

[alexgg]

These changes only refer to the raspberrypicm4-ioboard-sb - even though some DTs tests have not passed I am merging to unblock related work.