lantiq: fix building AVM/EVA sysupgrade images on NOR devices

lantiq: fix building AVM/EVA sysupgrade images on NOR devices

Device/AVM unconditionally inherits Device/NAND, which changes the
sysupgrade image to sysupgrade-tar (instead of a "raw" sysupgrade image
as used in the Device/Default template).
This moves the Device/NAND call to the FRITZ3370 device definition, as
this is the only AVM/EVA device that uses NAND.
Additionally we have to define the IMAGE_SIZE for the AVM/EVA NOR
devices to ensure that the images are not discarded.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>