lib/oe/path.py: Add relsymlink()

Adds API to make a relative symbolic link between two directories. The
arguments are the same as oe.path.symlink()

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>