libuv for Unikraft

This is the port of libuv as a Unikraft external library. It depends on the following libraries that need to be added to Makefile in this order:

• pthreads, e.g. pthread-embedded
• libc, e.g. newlib
• `lwip'

Please refer to the README.md as well as the documentation in the doc/ subdirectory of the main unikraft repository.