pmb/flasher: Fix regression with exporting odin images (#1529)

This makes the flasher work when no flash_method attribute exists on
the args variable, which happens when it is invoked through the
pmbootstrap export --odin command. This is a regression introduced
in b29cc877

This fixes #1527

pmb/flasher/init.py

@@ -23,7 +23,11 @@ import pmb.helpers.mount
 
 def init(args):
     # Validate method
-    method = args.flash_method or args.deviceinfo["flash_method"]
+    if hasattr(args, 'flash_method'):
+        method = args.flash_method or args.deviceinfo["flash_method"]
+    else:
+        method = args.deviceinfo["flash_method"]
+
     if method not in pmb.config.flashers:
         raise RuntimeError("Flash method " + method + " is not supported by the"
                            " current configuration. However, adding a new flash method is "