Redo fix with just backspace key support

The last fix (now reverted) apparently broke all the keys. Focusing on
the bug report, we only need to fix the backspace key.

Author: komacke

ESPHamClock/ArduinoLib/Adafruit_RA8875.cpp

@@ -2871,7 +2871,7 @@ void Adafruit_RA8875::kbThread ()
 	    if (nr == 1) {
                 // arrow keys need a state machine to parse ESC [ A/B/C/D, plus non-block for normal ESC
                 ::printf ("KB: %d %c\n", buf[0], buf[0]);
-                if (isprint(buf[0])) {
+                if (isprint(buf[0]) || buf[0] == 8 || buf[0] == 127) {
                     pthread_mutex_lock (&kb_lock);
                         KBState &ks = kb_q[kb_qtail];
                         ks.c = buf[0];