scripts/oe-setup-builddir: do not read TEMPLATECONF from templateconf.cfg

Also, write to it only if it does not already exist.

As discussed here[1], reading from templateconf.cfg serves no purpose:
it exists only if the build directory has already been initialized, and so
the scripts will not change anything in the build directory anyway.

My adjustment is to keep the file however; it is useful as a pointer
to the original template, which can be utilized by config management
tools in the future.

[1] https://lists.openembedded.org/g/openembedded-architecture/topic/template_handling_in_oe_core/93968540



Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>