Add support for multi kernel Raspberry Pi machines

[agherzan]

This introduces two new machines:

• raspberrypi-armv7.conf aims at supporting all the ARMv7 Raspberry Pi boards
• raspberrypi-armv8.conf aims at supporting all the ARMv8 Raspberry Pi boards.

This initial support was tested on:

• Raspberry Pi 4 Model B
• Raspberry Pi 3 Model B
• Raspberry Pi 0 2 WiFi

Tests:

• Boot
• WiFi
• BT
• Modules load

Known broken/unsupported

• u-boot support

Landed in oe-core/master PR is in draft mode due to pending oe-core changes to correctly handle depmod-ing multiple kernel module sets:

• https://lists.openembedded.org/g/openembedded-core/message/170505
• https://lists.openembedded.org/g/openembedded-core/message/170506
• https://lists.openembedded.org/g/openembedded-core/message/170507
• https://lists.openembedded.org/g/openembedded-core/message/170508

[zyga]

@agherzan did you test booting with u-boot as well as pi-boot?

[agherzan]

@zyga There should be a comment in the code but I'll make this clearer in the description (no, no support tested with uboot for now - it would be a somehow easy next step).