Add FlixPeriph library (peripheral drivers for Flix educational drone) (arduino#4215)

Author: okalachev

repositories.txt

@@ -4229,6 +4229,7 @@ https://github.com/ohad32/FireBase32
 https://github.com/ojx/GigaAudio
 https://github.com/ojx/HTTPed
 https://github.com/okalachev/mavlink-arduino
+https://github.com/okalachev/flixperiph
 https://github.com/OkbaO/KS0108_LCD
 https://github.com/OkbaO/ST7565_LCD
 https://github.com/OladapoAjala/Drive