[Buildroot] Replace libev with libuv

[Nimai Mahajan]

libuv supercedes libev in many ways, adds more features, and is
regularly updated. libev has not been updated for more than a year
now. libuv uses autogen.sh which is tricky to get working properly in
Buildroot so I request the higher authorities to add this as it is
perhaps the most useful asynchronous I/O library in C. Current
unstable version is 0.11.26 and stable is 0.10.27. Thank you.