Add possibility of signing with external keys in mender-convert

Acceptance criteria:

• Mender-convert modified to call this command if at least one of the two variables MENDER_SBSIGN_KEY and MENDER_MOKSIGN_KEY is set. Use the variables as arguments to the command.
• A full boot test is not required if this has already been solved for Ubuntu signed binaries. But a test which tests that the shim, grub, kernel, modules, and modules inside the initrd, have been signed, is required.