Add custom backend URL support (#28)

Add support of a custom OTA update service that runs the Thistle
procotol. If no custom backend URL is provided, the default Thistle
backend will be used.

action.yml

@@ -45,6 +45,10 @@ inputs:
     description: 'Password for the signing key. Required only if signing_key_management is "local" and the signing key is password protected'
     required: false
     default: ''
+  backend_url:
+    description: 'URL of the backend server that runs the OTA update service. If not provided, the Thistle service will be used'
+    required: false
+    default: ''
 runs:
   using: 'docker'
   image: 'Dockerfile'

entrypoint.sh

@@ -19,6 +19,7 @@
 # INPUT_SIGNING_KEY_MANAGEMENT
 # INPUT_SIGNING_KEY
 # INPUT_SIGNING_KEY_PASSWORD
+# INPUT_BACKEND_URL
 #
 
 set -euxo pipefail
@@ -34,6 +35,7 @@ readonly TRH_DOWNLOAD_URL
 export THISTLE_KEY="${HOME}/.minisign/minisign.key"
 export THISTLE_KEY_PASS="${INPUT_SIGNING_KEY_PASSWORD}"
 export THISTLE_TOKEN="${INPUT_PROJECT_ACCESS_TOKEN}"
+[ -z "${INPUT_BACKEND_URL}" ] || export THISTLE_BACKEND="${INPUT_BACKEND_URL}"
 
 err() {
   echo -e "$*"