x86: conga: Add option to select different config headers for baseboards

This patch adds the infrastructure to define different config headers
with different configurations and default environment for the baseboards
that can now be selected via Kconfig. The new configuration for the
theadorable-x86-conga-qa3-e3845 is also added. Also the new defconfig
file for this new target is added.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>