A Digitally Signed Boot is essentially a process where the bootloader performs a check on any new CE build being loaded onto into Flash in order to verify whether this build meets certain required characteristics.
However, this process is not currently supported by any of our bootloaders.
In order to move forward here, it would be necessary to know what exactly you are hoping to accomplish by using a Digitally Signed Boot. Feel free to contact ADS Support directly by emailing us at support.us@eurotech.com