bcm27xx: switch from ext4 to squashfs

Similar to x86 build images with a squashfs instead of ext4 filesystem
on bcm27xx / for the Raspberry Pi. This allows to use sysupgrade
properly and allows to use the firstboot command.

Runtime tested on a Raspberry Pi 4. Build tested for the Pi 3 / bcm2710.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>