* guix-kernel/hmac.scm (get-signature): Rewrite using (gcrypt hmac)
and (gcrypt base16).
* configure.ac: Check for (gcrypt hmac).
* environment.scm: Remove 'sed-openssl' phase; add GUILE-GCRYPT to DEPS
in the 'sed-kernel-json' phase.  Add GUILE-GCRYPT to 'propagated-inputs'.