oe-setup-builddir: Keep templateconf.cfg relative if $TEMPLATECONF is

oe-setup-builddir: Keep templateconf.cfg relative if $TEMPLATECONF is

Before commit 7b96dc80 (scripts/oe-setup-builddir: write to
conf/templateconf.cfg after the build is set up), the path written to
templateconf.cfg was relative if $TEMPLATECONF was relative, but
afterwards it became absolute.

Restore the original behavior of saving the relative path to
templateconf.cfg.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>