Add option to enable LTO easily (qmk#5674)

* Add option to enable LTO easily and disable features that cause compiling errors with LTO

* Add documentation about LTO option

* Add to show_options

docs/config_options.md

@@ -330,6 +330,8 @@ Use these to enable or disable building certain features. The more you have enab
  * Forces the keyboard to wait for a USB connection to be established before it starts up
 * `NO_USB_STARTUP_CHECK`
  * Disables usb suspend check after keyboard startup. Usually the keyboard waits for the host to wake it up before any tasks are performed. This is useful for split keyboards as one half will not get a wakeup call but must send commands to the master.
+* `LINK_TIME_OPTIMIZATION_ENABLE`
+ = Enables Link Time Optimization (`LTO`) when compiling the keyboard. This makes the process take longer, but can significantly reduce the compiled size (and since the firmware is small, the added time is not noticable). However, this will automatically disable the old Macros and Functions features automatically, as these break when `LTO` is enabled. It does this by automatically defining `NO_ACTION_MACRO` and `NO_ACTION_FUNCTION` 
 
 ## USB Endpoint Limitations
 

show_options.mk

@@ -88,7 +88,8 @@ OTHER_OPTION_NAMES = \
  RGB_MATRIX_KEYPRESSES \
  LED_MIRRORED \
  RGBLIGHT_FULL_POWER \
- Link_Time_Optimization
+ Link_Time_Optimization \
+ LINK_TIME_OPTIMIZATION_ENABLE
 
 define NAME_ECHO
  @echo " $1 = $($1) # $(origin $1)"

tmk_core/common.mk

@@ -208,6 +208,13 @@ ifeq ($(strip $(SHARED_EP_ENABLE)), yes)
  TMK_COMMON_DEFS += -DSHARED_EP_ENABLE
 endif
 
+
+ifeq ($(strip $(LINK_TIME_OPTIMIZATION_ENABLE)), yes)
+ EXTRAFLAGS += -flto
+ TMK_COMMON_DEFS += -DLINK_TIME_OPTIMIZATION_ENABLE
+ TMK_COMMON_DEFS += -DNO_ACTION_MACRO
+ TMK_COMMON_DEFS += -DNO_ACTION_FUNCTION
+endif
 # Bootloader address
 ifdef STM32_BOOTLOADER_ADDRESS
  TMK_COMMON_DEFS += -DSTM32_BOOTLOADER_ADDRESS=$(STM32_BOOTLOADER_ADDRESS)