simple-init: allow disabling DHCP fallback

In some cases this is not desired (see the dependecy for details).

Change-Id: Ie549d0c9769a2b6c58de9e61cb621ca96c1cd886
Depends-On: https://review.opendev.org/c/opendev/glean/+/781500

diskimage_builder/elements/simple-init/README.rst

@@ -68,3 +68,12 @@ non-zero.  See the glean documentation for what the implications for
 this are on each platform.
 
 This is currently only implemented for CentOS and Fedora platforms.
+
+Fallback
+--------
+
+Glean falls back to DHCPv4 for all interfaces that do not have an explicit
+configuration. If this is not desired, set the following variable to disable
+the fallback and leave such interfaces unconfigured::
+
+    export DIB_SIMPLE_INIT_NO_DHCP_FALLBACK=1

diskimage_builder/elements/simple-init/install.d/50-simple-init

@@ -29,4 +29,9 @@ if [[ ${DIB_SIMPLE_INIT_NETWORKMANAGER} != 0 ]]; then
     enable_nm="--use-nm"
 fi
 
-glean-install ${enable_nm}
+no_dhcp_fallback=""
+if [[ ${DIB_SIMPLE_INIT_NO_DHCP_FALLBACK:-0} != 0 ]]; then
+    no_dhcp_fallback="--no-dhcp-fallback"
+fi
+
+glean-install ${enable_nm} ${no_dhcp_fallback}