Better non-Linux support (#2524)

* grep -> $(GREP)

Some UNIXy systems (FreeBSD for example) don't use GNU grep by default.
Allow the user to specify which grep implementation to use so that
GNU grep can be specified.

* Allow using versioned avr-gcc command

Don't hardcode "avr-gcc", and allow strings such as "avr-gcc8", or
"avr-gcc-7.3.0" to match checks for "avr-gcc".

keyboards/handwired/woodpad/rules.mk

@@ -71,8 +71,8 @@ RGBLIGHT_ENABLE ?= no
 #    while [ -z $$USB ]; do \
 #      sleep 1; \
 #      ls /dev/tty* > /tmp/2; \
-#      USB=`diff /tmp/1 /tmp/2 | grep -o '/dev/tty.*'`; \
+#      USB=`diff /tmp/1 /tmp/2 | $(GREP) -o '/dev/tty.*'`; \
 #    done; \
 #    avrdude -p $(MCU) -c avr109 -P $$USB -U flash:w:$(BUILD_DIR)/$(TARGET).hex
 #
-#.PHONY: avrdude
+#.PHONY: avrdude