Switch to package.maintscript

This removes the following lintian warning:
W: raspi3-firmware: maintainer-script-should-not-use-dpkg-maintscript-helper

By adding a raspi3-firmware.maintscript that contains all the command
that are supposed to be passed to dpkg-mainscript-helper. This also
refactorizes code between postinst,postrm and preinst scripts