Fix case-insensitive ESPDuino file name

I work on Linux and I got an error about a missing `pins_arduino.h`:
```
fatal error: pins_arduino.h: No such file or directory
```
As it turns out, the reason for this is that the directory name under Linux is case-sensitive while it is lowercase-only in the variants file.

Author: ulikoehler

boards.txt

@@ -248,7 +248,7 @@ espduino.build.mcu=esp8266
 espduino.build.f_cpu=80000000L
 espduino.build.board=ESP8266_ESP13
 espduino.build.core=esp8266
-espduino.build.variant=espduino
+espduino.build.variant=ESPDuino
 espduino.build.flash_mode=dio
 espduino.build.flash_size=4M
 espduino.build.flash_freq=40