scripts/oe-setup-builddir: add a check that TEMPLATECONF is valid

scripts/oe-setup-builddir: add a check that TEMPLATECONF is valid

specifically that ../../layer.conf exists, and that second-from-last
component in the path is 'templates'.

This requires tweaking template.conf creation in eSDK bbclass, as
we need to ensure that the path in it is valid, and exists
(which may not be the case if the SDK is poky-based).

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