libelf for Unikraft

This is a port of libelf from official ELF Tool Chain 0.7.1

Project: https://sourceforge.net/projects/elftoolchain/ Sources: https://downloads.sourceforge.net/project/elftoolchain/Sources/elftoolchain-0.7.1/elftoolchain-0.7.1.tar.bz2

Further information

Further information about libelf can be found in README.elftoolchain. Please refer to the README.md as well as the documentation in the doc/ subdirectory of the main unikraft repository.