Add platformio configuration.

[bertrik]

This merge request add a platformio.ini file.

Platformio allows you to build the source code from the command line, automatically downloading the required toolchains, libraries and upload tools. This allows you, for example, to run an automated compile on travis-ci for example. It also takes the guesswork out of configuring the Arduino IDE, as things like the hardware, OTA configuration, library versions can all be specified.

You can install it on Linux as follows (example for debian):
sudo apt-get install python-pip
sudo pip install platformio
Then compile the source code & upload the binary by running
pio run -t upload

[1technophile]

you need to add esp32 ble library also

[bertrik]

The esp32 BLE library should already be included with the esp32 development environment that's downloaded by platformio, see also the README at https://github.com/nkolban/ESP32_BLE_Arduino

For me, it compiles and links fine like this.

[1technophile]

You are right, it is already provided.