Support building with `make HYPERVISOR=firecracker`.
Notes:
- Firecracker is currently only supported on x86_64, hence the
architecture check.
- The template file `cli/config/configuration-fc.toml.in` is based
on `cli/config/configuration.toml.in`.
Fixes#1105.
Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>