package/nftables: add an option to force JSON support

Currently, nftables JSON support is implicit: if jansson is enabled,
JSON support is enabled, otherwise it is not.

Some packages will require that nftables be built with JSON support;
that's the case of netavarks, for example (to come in a later patch).
So, in the current state, it would mean a package would have to select
jansson, although it does not itself use it, so that nftables has JSON
support. This does not feel right.

Instead, introduce an option to nftables, to explicitly enabled JSON
support. Packages such as netavark would then just have to require JSON
support in nftables.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/nftables/Config.in

@@ -13,9 +13,14 @@ config BR2_PACKAGE_NFTABLES
 
 	  http://www.netfilter.org/projects/nftables/index.html
 
-# Legacy: this used to be handled in nftables.mk
 if BR2_PACKAGE_NFTABLES
+# Legacy: this used to be handled in nftables.mk
 source "package/nftables/nftables-python/Config.in"
+
+config BR2_PACKAGE_NFTABLES_JSON
+	bool "json support"
+	select BR2_PACKAGE_JANSSON
+
 endif
 
 comment "nftables needs a toolchain w/ wchar, headers >= 3.12"

package/nftables/nftables.mk

@@ -42,7 +42,7 @@ else
 NFTABLES_CONF_OPTS += --without-cli
 endif
 
-ifeq ($(BR2_PACKAGE_JANSSON),y)
+ifeq ($(BR2_PACKAGE_NFTABLES_JSON),y)
 NFTABLES_DEPENDENCIES += jansson
 NFTABLES_CONF_OPTS += --with-json
 else