Makefile: Tweaks to install target; variable names

- Switch from "d" to "DESTDIR" to better align with convention.
- Rename "INSTALLER" to "SUITE", which makes more sense.
- Add a target for DESTDIR which errors out if it doesn't exist.
- Modify the install target to depend on the DESTDIR target
- Cause the build to fail if DESTDIR isn't writable
- Add "-f" to "rm debathena.preseed" in clean
- Standardize on parens for variable expansion
- Simplify the arch-specific "host" target.
- Add LIBEXECDIR to define where in the tarball binaries go,
  and add a target for it (and roll the kexec/host targets
  into it)
- Drop some unused phony targets