Install ndisc6 package in element script

The ndisc6 package is not yet available in EPEL 8.
See: https://bugzilla.redhat.com/show_bug.cgi?id=1779134

Until the package is available set the pkg-map to "" for
the ndisc6 package when distro is redhat and install the
package using || true in the element script instead so
that CentOS 8 build's do not fail because of the missing
package.

Once the package is in EPEL 8 this change can be reverted.

Related-Bug: #1754219
Change-Id: Icd4bad8852ce5ba40fb0e7b0d335191efbe88c67

diskimage_builder/elements/dhcp-all-interfaces/install.d/50-dhcp-all-interfaces

@@ -8,6 +8,17 @@ set -o pipefail
 
 SCRIPTDIR=$(dirname $0)
 
+# TODO(hjensas): Once ndisc6 package is available in EPEL8 drop this, and add the package back in pkg-map.
+# See: https://bugzilla.redhat.com/show_bug.cgi?id=1779134
+if [ -e "/etc/redhat-release" ]; then
+    if type dnf &>/dev/null; then
+        dnf -v -y install ndisc6 || true
+    else
+        yum -v -y install ndisc6 || true
+    fi
+fi
+
+
 # this script is not needed on Gentoo.
 if [ "$DISTRO_NAME" != "gentoo" ]; then
     install -D -g root -o root -m 0755 ${SCRIPTDIR}/dhcp-all-interfaces.sh /usr/local/sbin/dhcp-all-interfaces.sh

diskimage_builder/elements/dhcp-all-interfaces/pkg-map

@@ -2,7 +2,7 @@
   "family": {
     "redhat": {
       "dhcp-client": "dhclient",
-      "ndisc6": "ndisc6"
+      "ndisc6": ""
     },
     "gentoo": {
       "dhcp-client": "net-misc/dhcpcd",