Forces user to set CoreOS device

Adds notation to ensure users are aware of
device to be overwritten as CoreOS expects ignition and
device set for baremetal installs

Closes: https://github.com/netbootxyz/netboot.xyz/issues/1579

roles/netbootxyz/templates/menu/coreos.ipxe.j2

@@ -11,14 +11,14 @@ set os {{ releases.coreos.name }}
 set os_arch ${arch}
 iseq ${os_arch} x86_64 && set os_arch x86_64 ||
 iseq ${os_arch} arm64 && set os_arch aarch64 ||
-isset ${install_device} || set install_device /dev/sda
+isset ${install_device} || set install_device unset
 isset ${ignition_url} || set ignition_url skip
 menu ${os} - ${os_arch}
 item --gap ${os}:
 {% for item in releases.coreos.versions %}
 item {{ item.code_name }} ${space} ${os} {{ item.name }}
 {% endfor %}
-item install_dev ${space} Set install device: ${install_device}
+item install_dev ${space} Set install device (e.g. /dev/sda): ${install_device}
 item ignition_config ${space} Set ignition config url: ${ignition_url}
 choose --default ${core_version} core_version || goto coreos_exit
 echo ${cls}
@@ -40,6 +40,8 @@ boot
 goto coreos_exit
 
 :install_dev
+echo -n Selected install device will be overwritten,
+echo -n please ensure you know what are you doing...
 echo -n Please set desired install device:  && read install_device
 clear menu
 goto coreos