makes 'ld' to overlook binary objects compatibility.

* Moved $(PLATFORM_LIBS) from the library group (--start-group ...
  --end-group) outside of the group. This will make 'ld' to do
  _multiple_ search in the library group when resolving symbol
  references and do only a _single_ seach in libgcc.a after the group
  search.

* Fix stability problems on CPC45 board again.

* Make image detection for diskboot / usbboot / scsiboot more robust
  (also check header checksum)