Fix build if DOWNLOAD is set, but no JSON

The downloader does not require JSON, but channel_curl is built
even if not called. Build fails with the error:

corelib/channel_curl.c:27:10: fatal error: json-c/json.h: No such file or directory

Add a CONFIG_CHANNEL_CURL that is automatically set by the modules
reuiring it (suricatta and swuforwarder).

Signed-off-by: Stefano Babic <sbabic@denx.de>
Reported-by: Jörg Krause <joerg.krause@embedded.rocks>
Acked-by: Jörg Krause <joerg.krause@embedded.rocks>

Kconfig

@@ -294,6 +294,13 @@ config DOWNLOAD
 comment "Image downloading support needs libcurl"
 	depends on !HAVE_LIBCURL
 
+config CHANNEL_CURL
+	bool
+	depends on HAVE_LIBCURL
+	depends on HAVE_JSON_C
+	select CURL
+	select JSON
+
 config HASH_VERIFY
 	bool "Allow to add sha256 hash to each image"
 	depends on HAVE_LIBSSL

corelib/Makefile

@@ -17,4 +17,4 @@ lib-$(CONFIG_ENCRYPTED_IMAGES)	+= swupdate_decrypt.o
 lib-$(CONFIG_LIBCONFIG)		+= swupdate_settings.o \
 				   parsing_library_libconfig.o
 lib-$(CONFIG_JSON)		+= parsing_library_libjson.o
-lib-$(CONFIG_CURL)		+= channel_curl.o
+lib-$(CONFIG_CHANNEL_CURL)	+= channel_curl.o

handlers/Config.in

@@ -54,7 +54,6 @@ config UBIVIDOFFSET
 	  if your NAND driver incorrectly reports that it can handle
 	  sub-page accesses when it should not.
 
-
 config CFI
 	bool "cfi"
 	depends on MTD
@@ -164,8 +163,8 @@ comment "remote handler needs zeromq"
 config SWUFORWARDER_HANDLER
 	bool "SWU forwarder"
 	depends on HAVE_LIBCURL
-	select CURL
-	select JSON
+	depends on HAVE_JSON_C
+	select CHANNEL_CURL
 	default n
 	help
 	  This allows to build a chain of updater. A
@@ -174,6 +173,9 @@ config SWUFORWARDER_HANDLER
 	  embedded SWU to the other devices using the
 	  Webserver REST API.
 
+comment "swuforward handler needs json-c and curl"
+	depends on !HAVE_JSON_C || !HAVE_LIBCURL
+
 comment "SWU forwarder requires libcurl"
 	depends on !HAVE_LIBCURL
 

suricatta/Config.in

@@ -71,8 +71,7 @@ config SURICATTA_HAWKBIT
 	bool "hawkBit support"
 	depends on HAVE_LIBCURL
 	depends on HAVE_JSON_C
-	select JSON
-	select CURL
+	select CHANNEL_CURL
 	help
 	  Support for hawkBit server.
 	  https://projects.eclipse.org/projects/iot.hawkbit