ConfHandlerr: Use full path in variable history

When an evaluation was made for a configuration file the path to the
file was saved as a relative one. The change in this commit will save the
location as an absolute path. This way the user will have full information
regarding the location of the file where a variable was changed and the
line withing the file.

[YOCTO #5562]

Signed-off-by: Marius Avram <marius.avram@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>