Fix erroneous relative path resolution

[ivanvig]

Currently FuseSoC has two problems related to relative path resolution,

The more severe one occurs when the fusesoc.conf file is in the current directory, in that case the code

files_read = self._cp.read(config_files)

returns only the file name, then in the function _resolve_path_from_cfg when os.path.dirname is called, an empty string is returned and when joined to form the final path we get a broken result

A second issue that I found is when referencing to a relative path starting with a dot, for example ./build_root that dot stays in the path producing a result like /abs/path/to/./build_root which although is a valid path, fails path comparisons, like the ones that are done for ignored_dirs

This PR includes tests for both cases and fixes them.