[Bug] Set max-firmware-size in EV3 to match u-boot.

[laurensvalk]

Describe the bug u-boot reads a fixed amount of space, but there isn't currently a check that our firmware isn't bigger than that.

Expected behavior We should set max-firmware-size in the metadata to reflect this, so we'll get an appropriate warning if the firmware is bigger.

Alternatives Make u-boot read exactly as much as needed by inspecting data in the image.