data: rename defaultval to _defaultval

data: rename defaultval to _defaultval

The defaultval field is intended to be internal and the only use of that field
outside of data.py is to skip over it when iterating over a value's flags.

For clarity and convenience, rename the field to _defaultval so that it is
considered internal and not exposed through the data API.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>