Add servo mode support for spindle PWM control

- Implemented USE_SPINDLE_SERVO_MODE configuration option
- Changed Timer2 prescaler from 1/8 to 1/1024 for ~50Hz servo frequency
- Added configurable servo pulse width range (SPINDLE_SERVO_MIN_PULSE/MAX_PULSE)
- M3 S0-S1000 now controls servo position (0-180 degrees)
- M5 positions servo to maximum position (pen up) instead of disabling PWM
- Default configuration supports pen plotter with reversed pulse values (MIN=31, MAX=16)
- Only supports ATmega328p (Arduino Uno) processors

Based on: https://www.buildlog.net/blog/2017/08/using-grbls-spindle-pwm-to-control-a-servo/

platformio.ini

@@ -0,0 +1,41 @@
+; PlatformIO Project Configuration File for Grbl
+;
+; This configuration builds Grbl v0.9j for Arduino Uno (ATmega328P)
+; Based on the original Makefile settings
+;
+; Please visit documentation for options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[platformio]
+src_dir = src
+lib_dir = .
+include_dir = grbl
+
+[env:uno]
+platform = atmelavr
+board = uno
+framework = arduino
+
+; Build configuration matching Makefile
+build_flags =
+    -DF_CPU=16000000L
+    -DBAUD_RATE=115200
+    -Wall
+    -Os
+    -ffunction-sections
+    -fdata-sections
+    -Wl,--gc-sections
+    -lm
+    -Igrbl
+    -Igrbl/cpu_map
+    -Igrbl/defaults
+
+; Add Grbl source files to build
+build_src_filter =
+    +<*>
+    +<../grbl/*.c>
+    -<../grbl/examples/>
+
+; Upload configuration
+upload_speed = 115200
+monitor_speed = 115200