Since heads passed to [musl-cross-make](https://github.com/osresearch/heads/commit/8e4b10922bcfa70eb1891e8933d5e4cb93191c00), it is impossible to sign /boot files under Heads. - [x] linux doesn't provide a working tty so pinentry doesn't permit to enter passphrase to sign with USB security dongle  - [x] pinentry-tty is not built against the cross compiler
Since heads passed to musl-cross-make, it is impossible to sign /boot files under Heads.