econet: sysupgrade: Prevent flashing old flash driver images

Output a message that a downgrade to versions < 7.1.0ALPHA3 is not
officially supported thus preventing a bricking the device with versions
7.x.x before 7.1.0ALPHA3 and versions 6.x.x before 6.5.3BETA2. This is
due to the different flash driver and flash layout used.

6.5.3BETA2 has an extra check in the upgrade bundle that prevents
flashing in this situation, but past images cannot be changed. Leverage
instead the fact that old FIT image had a component "squashfs" that is
no longer present in the new images.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>