kconfig: support custom server certs

when a custom server address is set using CONFIG_GOLIOTH_COAP_HOST_URI,
cmake compares the setting to the expected values for prod/dev and includes
the server certificate automatically. This caused the build to fail when a
custom address is selected because no certificate will be included in the
build.

add CONFIG_COAP_SERVER_CERT_PEM_LOCATION which may be set to the location
of a server cert in PEM format. This is optional when using the addresses
of the prod/dev server, but required for all other
CONFIG_GOLIOTH_COAP_HOST_URI values.

Signed-off-by: Mike Szczys <mike@golioth.io>

Author: szczys

Kconfig

@@ -23,6 +23,18 @@ config POUCH_GATEWAY_SERVER_CERT_MAX_LEN
     help
       Maximum length of server certificate.
 
+config COAP_SERVER_CERT_PEM_LOCATION
+    string "Custom CoAP Server Cert Location"
+    default ""
+    help
+
+        By default, the gateway build automatically includes a
+        certificate for prod/dev when CONFIG_GOLIOTH_COAP_HOST_URI
+        matches the known URI for either server. When a different URI
+        is set (or a different server certificate is needed) this
+        symbol must be set to the absolute location of the server cert
+        PEM file.
+
 config POUCH_GATEWAY_DOWNLINK_BLOCK_TIMEOUT
     int "Downlink timeout"
     default 10

lib/CMakeLists.txt

@@ -19,10 +19,18 @@ zephyr_library_link_libraries(mbedTLS)
 
 zephyr_library_get_current_dir_lib_name(${ZEPHYR_BASE} lib_name)
 
-if(CONFIG_GOLIOTH_COAP_HOST_URI STREQUAL "coaps://coap.golioth.io")
+if (NOT (CONFIG_COAP_SERVER_CERT_PEM_LOCATION STREQUAL ""))
+  set(cert_name ${CONFIG_COAP_SERVER_CERT_PEM_LOCATION})
+elseif(CONFIG_GOLIOTH_COAP_HOST_URI STREQUAL "coaps://coap.golioth.io")
   set(cert_name "server-prod.pem")
 elseif(CONFIG_GOLIOTH_COAP_HOST_URI STREQUAL "coaps://coap.golioth.dev")
   set(cert_name "server-dev.pem")
+else()
+    message(SEND_ERROR
+        "Unable to find server certificate for CONFIG_GOLIOTH_COAP_HOST_URI value \
+        (${CONFIG_GOLIOTH_COAP_HOST_URI}). Set CONFIG_COAP_SERVER_CERT_PEM_LOCATION to PEM file \
+        location.
+    ")
 endif()
 
 generate_inc_file_for_target(${lib_name}