net: lib: downloader: make sure at least one transport enabled

The downloader library require at least one transport to be enabled.
Add build assert when downloader library is enabled without any
transport.

Signed-off-by: Eivind Jølsgard <[email protected]>

Author: eivindj-nordic

subsys/net/lib/downloader/dl_transports.ld

@@ -3,3 +3,6 @@
 _dl_transport_entry_list_start = .;
 KEEP(*(SORT_BY_NAME("._dl_transport_entry.*")));
 _dl_transport_entry_list_end = .;
+
+ASSERT(_dl_transport_entry_list_end != _dl_transport_entry_list_start,
+       "No transport enabled for the downloader library. See downloader library Kconfig options.");