ipq806x: Increase kernel size to 4 MB for EA8500/EA7500v1

ipq806x: Increase kernel size to 4 MB for EA8500/EA7500v1

Increase the kernel size from 3 MB to 4 MB for EA8500 and EA7500v1.
* modify the common .dtsi
* modify the kernel size in the image recipes

Define compat-version 2.0 to force factory image usage for sysupgrade.
Add explanation message. Reenable both devices.

As for 4MiB (and not more): Hannu Nyman noted that:
"We have lots of ipq806x devices with 4 MB kernel, so will
need action at that point in future in any case.
(Assuming that the bootloader did not have a 4 MB limit that
has been tested...)"

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
(squashed, added 4MiB notice of support in ipq806x)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>