Remove makefile and do special steps from an override-build section
instead.
Take now signed grub and shim form the ucdev/uc-staging-ppa PPA, using
package-repositories. Now we use epochs in the version of
grub-efi-amd64-signed and shim-signed packages, so the build should
always take the ones from the PPA instead of those in the archive, as
2:<version> will always be greater than the package version in the
latter.
snapcraft.yaml: allow building outside of launchpad with old binaries
The measurements in 2.06 grub are currently incorrect, force using
focal binaries, without requirement to specify archive in launchpad
/+snap/ page such that one can build the gadget locally correctly too.
Also remove all unused dependencies. And it is no longer required to
stage build-deps, instead of build-depend on them, as they are no
longer conflicting with anything installed on the host.
Fixes #57
Backport from 00ea33892ff0b258d2e4136d7b911a9c8ce0605d
