libwebsock: new package

[Thomas:
 - Add dependency on BR2_TOOLCHAIN_HAS_THREADS
 - Improve Config.in help text
 - Fixup comment header in .mk file.
 - Use github helper.
 - Remove hash file (we're downloading from Git)
 - Fixup LICENSE_FILES
 - Remove LIBWEBSOCK_CFLAGS, they were the default.
 - Remove LIBWEBSOCK_LIBS, and instead use a patch to switch
   libwebsock configure script to use pkg-config.]

Signed-off-by: Gregory Dymarek <gregd72002@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>