Add define for ePulse Feather VBAT Monitoring Voltage Divider

[matthias-bs]

Problem

While the Thingpulse ePulse Feather – Low Power ESP32 development board is (mostly) pin compatible to the Adafruit HUZZAH32 - ESP32 Feather, the VBAT monitoring voltage divider is different:

ePulse Feather

Huzzah32 ESP32 Feather

100k/100k (1:1 divider - as all other development boards seen so far)

Workaround

Since there is no "Thingpulse ePulse Feather" board definition in the Arduino ESP32 Board Package (AFAIK), users would typically select "Adafruit ESP23 Feather" instead.

A define has to be added to the sketch to distinguish the two boards accordingly.

[matthias-bs]

@marcelstoer: For your kind attention.

[matthias-bs]

espressif/arduino-esp32#9256