mkimage: allow internalization of data-position

Make it possible for data that was externalized using a static external
position (-p) to be internalized. Enables the ability to convert
existing FIT images built with -p to be converted to a FIT image where the
data is internal, to be converted to a FIT image where the data is
external relative to the end of the FIT (-E) or change the initial
static external position to a different static external position (-p).

Removing the original external-data-related properties ensures that
they're not present after conversion. Without this, they would still be
present in the resulting FIT even if the FIT has been, for example,
internalized.

Signed-off-by: Lars Feyaerts <lars@bitbiz.be>
Reviewed-by: Simon Glass <sjg@chromium.org>