__bird_static: update to use `channel` param instead of `ipv4` switch.

type/__bird_static/files/template.sh

@@ -13,12 +13,7 @@ printf "protocol static %s {\n" "${__object_id:?}"
 [ -n "${description?}" ] && printf "\tdescription \"%s\";\n" "${description:?}"
 
 # Channel choice
-if [ -f "${__object:?}/parameter/ipv4" ];
-then
-	printf "\tipv4;\n"
-else
-	printf "\tipv6;\n"
-fi
+printf "%s;\n" "$(cat "${__object:?}/parameter/channel")"
 
 # Routes
 while read -r route

type/__bird_static/man.rst

@@ -16,6 +16,11 @@ used unless the `ipv4` flag is passed. This type **expects** to depend on the
 `cdist-type__bird_core(7)` type.
 
 
+REQUIRED PARAMETERS
+-------------------
+channel
+   The channel to use between the protocol and the table.
+
 REQUIRED MULTIPLE PARAMETERS
 ----------------------------
 route
@@ -28,11 +33,6 @@ OPTIONAL PARAMETERS
 description
     An instance desciption to be printed when `birdc show protocols` is called.
 
-BOOLEAN PARAMETERS
-------------------
-ipv4
-   Use the ipv4 channel instead of the default ipv6 one.
-
 
 EXAMPLES
 --------

type/__bird_static/parameter/boolean

@@ -1 +0,0 @@
-ipv4

type/__bird_static/parameter/required

@@ -0,0 +1 @@
+channel