projectconfig.yaml: Add key for default targets

_versions.py: Bump format version

buildstream/_versions.py

@@ -23,7 +23,7 @@
 # This version is bumped whenever enhancements are made
 # to the `project.conf` format or the core element format.
 #
-BST_FORMAT_VERSION = 20
+BST_FORMAT_VERSION = 21
 
 
 # The base BuildStream artifact version

buildstream/data/projectconfig.yaml

@@ -167,3 +167,11 @@ shell:
   # Command to run when `bst shell` does not provide a command
   #
   command: [ 'sh', '-i' ]
+
+# Defaults for bst commands
+#
+defaults:
+
+  # Set default target elements to use when none are passed on the command line.
+  # If none are configured in the project, default to all project elements.
+  targets: []