examples/gnrc_border_router: remove BOARD_BLACKLIST variable
examples/gnrc_border_router: remove BOARD_BLACKLIST variable

pic32 boards now provide an UART, and this the periph_uart feature
ruuvitag/thingy52 provide the periph_uart feature so stdio_uart can work
on these boards.

RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,toonst/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,mtausig/RIOT,toonst/RIOT,smlng/RIOT,kYc0o/RIOT,x3ro/RIOT,OlegHahm/RIOT,smlng/RIOT,authmillenon/RIOT,toonst/RIOT,basilfx/RIOT,basilfx/RIOT,basilfx/RIOT,yogo1212/RIOT,ant9000/RIOT,authmillenon/RIOT,yogo1212/RIOT,mtausig/RIOT,miri64/RIOT,x3ro/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,mtausig/RIOT,yogo1212/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,x3ro/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,yogo1212/RIOT,OTAkeys/RIOT,smlng/RIOT,OTAkeys/RIOT,kYc0o/RIOT,x3ro/RIOT,jasonatran/RIOT,basilfx/RIOT,smlng/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,OTAkeys/RIOT,yogo1212/RIOT,mtausig/RIOT,miri64/RIOT,mtausig/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,toonst/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,smlng/RIOT,jasonatran/RIOT,OTAkeys/RIOT,yogo1212/RIOT,miri64/RIOT,toonst/RIOT,basilfx/RIOT,x3ro/RIOT