lib: implemented oe.path.realpath()

lib: implemented oe.path.realpath()

Various parts of the buildsystem have to work with symlinks. Resolving
them is not trivial because they are always relative to a sysroot
directory.

Patch adds a function which returns the destination of a symlink by
assuming a given path as the / toplevel directory.  A testsuite was
added too.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>