Expand bootloader target to support most AVR boards (#6255)

* Update the :bootloader target to pass along correct hardware info

* Update make scripts to properly grab the settings (a big thanks to @yanfali)

* Remove LUFA debug warnings

lib/lufa/Bootloaders/DFU/makefile

@@ -11,11 +11,11 @@
 
 # Run "make help" for target help.
 
-MCU          = atmega32u4
-ARCH         = AVR8
-BOARD        = QMK
-F_CPU        = 16000000
-F_USB        = $(F_CPU)
+MCU          ?= atmega32u4
+ARCH         ?= AVR8
+BOARD        ?= QMK
+F_CPU        ?= 16000000
+F_USB        ?= $(F_CPU)
 OPTIMIZATION = s
 TARGET       = BootloaderDFU
 SRC          = $(TARGET).c Descriptors.c BootloaderAPI.c BootloaderAPITable.S $(LUFA_SRC_USB)
@@ -26,8 +26,8 @@ LD_FLAGS     = -Wl,--section-start=.text=$(BOOT_START_OFFSET) $(BOOT_API_LD_FLAG
 # Flash size and bootloader section sizes of the target, in KB. These must
 # match the target's total FLASH size and the bootloader size set in the
 # device's fuses.
-FLASH_SIZE_KB         = 32
-BOOT_SECTION_SIZE_KB  = 4
+FLASH_SIZE_KB         ?= 32
+BOOT_SECTION_SIZE_KB  ?= 4
 
 # Bootloader address calculation formulas
 # Do not modify these macros, but rather modify the dependent values above.