Use single brackets for portability with other shells

keyboards/lets_split/rules.mk

@@ -78,7 +78,7 @@ CUSTOM_MATRIX = yes
 avrdude: build
 	ls /dev/tty* > /tmp/1; \
 	echo "Reset your Pro Micro now"; \
-	while [[ -z $$USB ]]; do \
+	while [ -z $$USB ]; do \
 	  sleep 1; \
 	  ls /dev/tty* > /tmp/2; \
 	  USB=`diff /tmp/1 /tmp/2 | grep -o '/dev/tty.*'`; \