rpi-eeprom-digest: Add an option to verify RSA signed files.
rpi-eeprom-digest: Add an option to verify RSA signed files.

Wrap the OpenSSL commands to verify the RSA signature in a .sig
file for a given image. This could be used to verify a signed
boot image or EEPROM image before attempting to install it.

raspberrypi/usbboot,raspberrypi/usbboot,raspberrypi/usbboot