ATTiny84s need a longer chip erase delay

Following along with how the Arduino IDE (v1.8.8) does it. I was seeing inconsistent fuse bit programming (when it includes an erase command). Upping the chip_erase_delay to 15000 uS made this work 100% of the time.

Author: Pete Lewis

avrdude_gpio.conf

@@ -10718,7 +10718,7 @@ part
      avr910_devcode   = 0x20;
      signature        = 0x1e 0x93 0x0c;
      reset            = io;
-     chip_erase_delay = 4500;
+     chip_erase_delay = 15000;
 
      pgm_enable       = "1 0 1 0  1 1 0 0    0 1 0 1  0 0 1 1",
                         "x x x x  x x x x    x x x x  x x x x";