qmk_firmware/keyboards/handwired/42/rules.mk

# MCU name
MCU = atmega32u4

# Processor frequency
F_CPU = 8000000

# Bootloader selection
BOOTLOADER = caterina

# Build Options
#   change yes to no to disable
#
BOOTMAGIC_ENABLE = no       # Enable Bootmagic Lite
MOUSEKEY_ENABLE = yes       # Mouse keys
EXTRAKEY_ENABLE = yes       # Audio control and System control
CONSOLE_ENABLE = no         # Console for debug
COMMAND_ENABLE = no         # Commands for debug and configuration
NKRO_ENABLE = yes           # Enable N-Key Rollover
BACKLIGHT_ENABLE = no       # Enable keyboard backlight functionality
RGBLIGHT_ENABLE = no        # Enable keyboard RGB underglow
AUDIO_ENABLE = no           # Audio output

BLUETOOTH_ENABLE = yes
BLUETOOTH_DRIVER = BluefruitLE