Make used sdkconfig file user-selectible

[maxgerhardt]

Per community post.

The problem (examplatory): One ESP-IDF project, with two environments in the platformio.ini, should compile different firmwares based on a different sdkconfig configuration. However, only ever the sdkconfig file in the project is used and one cannot select a different sdkconfig file to be used during the build of each firmware.

Untested workaround: Write a small extra_script which copies the correct sdkconfig source file, (e.g. sdkconfig_cfg1) as sdkconfig, then continue building.

Feature request: Make used sdkconfig file user-selectible. Currently the espidf.py builder code uses a hardcoded (project dir)/sdkconfig string.