network/ifup.sh: default to dhcp for BOOTIF

modules.d/40network/ifup.sh

@@ -213,6 +213,13 @@ if [ "$netif" = "$vlanname" ] && [ ! -e /tmp/net.$vlanname.up ]; then
     ip link add dev "$vlanname" link "$phydevice" type vlan id "$(get_vid $vlanname; echo $?)"
 fi
 
+# BOOTIF= defaults to dhcpv4
+bootif=$(getargs BOOTIF=)
+if [ -n "$bootif" ] ; then
+    do_dhcp -4
+    exit 0
+fi
+
 # Specific configuration, spin through the kernel command line
 # looking for ip= lines
 for p in $(getargs ip=); do