kea: add support synthesizing imported config files

Configuring Kea JSON files is not trivial, and this might impede
the adoption of Kea as a DHCP server.  There are, however, many
users who used its predecessor ISC-DHCP, at least for DHCPv4.
A filter could ingest the legacy UCI and synthesize a JSON config
file for Kea DHCPv4.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

net/kea/Makefile

@@ -10,7 +10,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=kea
 PKG_VERSION:=3.0.2
-PKG_RELEASE:=4
+PKG_RELEASE:=5
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=https://ftp.isc.org/isc/kea/$(PKG_VERSION)
@@ -260,6 +260,8 @@ define Package/kea-uci/install
 	$(INSTALL_DIR) $(1)/etc/config $(1)/etc/init.d
 	$(INSTALL_CONF) ./files/kea.config $(1)/etc/config/kea
 	$(INSTALL_BIN) ./files/kea.init $(1)/etc/init.d/kea
+	$(INSTALL_DIR) $(1)/usr/lib/kea/importers
+	$(INSTALL_BIN) ./files/dhcp4.sh $(1)/usr/lib/kea/importers/
 endef
 
 define Package/kea-uci/conffiles

net/kea/files/kea.init

@@ -4,6 +4,7 @@ USE_PROCD=1
 START=25
 STOP=85
 
+LIB_PATH="/usr/lib/kea"
 BIN_PATH="/usr/sbin"
 CONF_PATH="/etc/kea"
 
@@ -16,6 +17,7 @@ start_service() {
 
 start_kea() {
 	local cfg="$1"
+	local name cmd cnf importer
 
 	config_get_bool disabled "$cfg" disabled 0
 	[ "$disabled" = "0" ] || return
@@ -24,9 +26,6 @@ start_kea() {
 
 	case "$name" in
 		ctrl_agent|dhcp4|dhcp6|dhcp_ddns)
-			name="${name/_/-}"
-			cmd="${BIN_PATH}/kea-${name}"
-			cnf="${CONF_PATH}/kea-${name}.conf"
 			;;
 		*)
 			echo "Error: Invalid service name '$name'" >&2
@@ -34,6 +33,18 @@ start_kea() {
 			;;
 	esac
 
+	name="${name/_/-}"
+	cmd="${BIN_PATH}/kea-${name}"
+	cnf="${CONF_PATH}/kea-${name}.conf"
+	importer="${LIB_PATH}/importers/${name}.sh"
+
+	if [ -f "$importer" ]; then
+		if ! "$importer" "$cnf"; then
+			echo "Error: Importer '$importer' failed; aborting." >&2
+			return 1
+		fi
+	fi
+
 	procd_open_instance "$name"
 	procd_set_param command "$cmd" -c "$cnf"
 	procd_set_param env KEA_LOCKFILE_DIR=/tmp