Ensure we have firmwares in the netinst image too

kali-config/variant-netinst/package-lists/kali.list.chroot

@@ -1,4 +1,8 @@
-# Empty list because netinst has no embedded packages
+# Mostly empty list because netinst has no embedded packages in theory
 #
 # This variant is not meant to be used with live-build but only with
 # simple-cdd and thus ./build.sh --installer --variant netinst
+
+# We still put kali-linux-core just to ensure that simple-cdd's mirror
+# contains the firmwares
+kali-linux-core