data.py: Add a warning when expandKeys overwrites an existing key

When two variables are defined as:

${var} = "bar"
foo = "foobar"

The value of 'foo' when ${var} == foo becomes indeterminate.  We
want to warn a user when this situation has been encountered so they
can take corrective actions.

In the above example usually foo == bar, unless multilibs are enabled.
Then ml-foo = "ml-foobar".

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>