Search for Kernel headers

Depending on the distribution, kernel headers might be in different
locations (newer Debian, *Ubuntu). This patch searches possible
locations.

Furthermore, uClibc needs files from /usr/include to be built correctly.
Merge pull request #5 from MartinNowack/feature_search_kernel_header

Search for Kernel headers
Add more log output when patching .config file for KERNEL_HEADERS
Use os.path.join instead of explicit string concatentation.