When the magic variable .KATI_READONLY is set to a variable name, any
further attempts to modify the named variable will result in an error.

  FOO := bar
  .KATI_READONLY := FOO
  FOO := baz  # Error!

This is useful to make some global configuration readonly so that
another makefile cannot change it. In Android, we emulated this by
backing up some global configuration before including the Android.mk
files, then comparing the current values to the backed up values after
they've been included. But this means we don't know the location that
modified the variable, just that something did. And it's not perfect,
since the backup can also be changed.

Something similar to this could be implemented with `override`, but then
setting the variable silently fails, and it still could be overriden
with another override.