Add Buxton data types

This commit adds the basic data types we will use in Buxton. There will be
no get/set_string type methods, just get/set_value, using the data struct
BuxtonData.

In order to use BuxtonData, you must set the BuxtonDataType to a valid type
such as string:
    BuxtonData data;
    data.type = STRING;

It is not desirable for backends to explicitly handle types, they should
store and return data per the requests in libbuxton. Buxton will then handle
sanity and verification of data + types.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>