Basicly ACRN scenario is a configuration name for specific usage. By giving
scenario name ACRN will load corresponding VM configurations to build the
hypervisor. But customer might have their own scenario name, change the
scenario type from choice to string is friendly to them since Kconfig source
file change will not be needed.
With this change, CONFIG_$(SCENARIO) will not exist in kconfig file and will
be instead of CONFIG_SCENARIO, so the Makefile need to be changed accordingly;
Tracked-On: #4616
Signed-off-by: Victor Sun <victor.sun@intel.com>