oe-setup-builddir: Always update templateconf.cfg

oe-setup-builddir: Always update templateconf.cfg

The intent of the templateconf.cfg file is to cache the value of
$TEMPLATECONF. To do this, it needs to be updated in case TEMPLATECONF
is manually specified.

Before, the following commands:

  TEMPLATECONF=foo . oe-init-build-env
  TEMPLATECONF=bar . oe-init-build-env
  . oe-init-build-env

would cause the templates in foo to be used for the last source of
oe-init-build-env, while afterwards, bar is used as expected.

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>