- remove BEGIN_FILE/END_FILE lines from output header file.
- add single disclaimer at the beginning of the file instead
of having it repeated for every array.
- improved exception message for missing key generation program.
This commits also regenerates "test_keys.h" in order to fully
comply with the new format.
Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>