image builder: allow passing commands directly by using % prefix

With this change you can hand-craft commands which normally would be
parsed by `image_builder.py`, eg:

%app flash0 -x psh;-i;/etc/rc.psh ddr ddr;nonexistent
(will not generate corresponding `alias`, will not add `psh` to the
target image automatically)

This usage is discouraged and should be used only in testing and some
extremely hackish use cases.

JIRA: CI-494

scripts/image_builder.py

@@ -133,6 +133,10 @@ def build(cls, cmd: str = "", **kwargs):
 
         # TODO: add compile-time checks for scripts validity (eg. memory regions cross-check)?
 
+        # allow passing commands directly without parsing them by using `%` prefix
+        if cmd.startswith("%"):
+            return PloCmdGeneric(cmd[1:].strip())
+
         # generic PLO command - treated for now as string
         return PloCmdGeneric(cmd)