libdfu: Add commands to encrypt and decrypt firmware images
libdfu: Add commands to encrypt and decrypt firmware images

This uses XTEA to encrypt and decrypt images in the firmware file, although
I'm open for adding other methods as required.

hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,vathpela/fwupd,superm1/fwupd,vathpela/fwupd,vathpela/fwupd,fwupd/fwupd,superm1/fwupd,fwupd/fwupd,vathpela/fwupd,superm1/fwupd