[Android Software Development] jcadduono:
I was bored and came up with this cool command to find you all the flags available for BoardConfig.mk when building TWRP:

grep -roh '\b\(TW\|TWRP\|RECOVERY\|TARGET\|BOARD\)_[_A-Z0-9]*' bootable/ --include=*.mk | sort | uniq

Here's a list of the 139 flags it found:

BOARD_BML_BOOT
BOARD_BML_RECOVERY
BOARD_CUSTOM_GRAPHICS
BOARD_HAS_FLIPPED_SCREEN
BOARD_HAS_LARGE_FILESYSTEM
BOARD_HAS_NO_REAL_SDCARD
BOARD_RECOVERY_BLDRMSG_OFFSET
BOARD_RECOVERY_DEFINES
BOARD_SEPOLICY_DIRS
BOARD_SEPOLICY_UNION
BOARD_UMS_LUNFILE
BOARD_USE_CUSTOM_RECOVERY_FONT
BOARD_USES_BML_OVER_MTD
RECOVERY_API_VERSION
RECOVERY_BUSYBOX_SYMLINKS
RECOVERY_BUSYBOX_TOOLS
RECOVERY_EXECUTABLES
RECOVERY_FSTAB_VERSION
RECOVERY_GRAPHICS_FORCE_USE_LINELENGTH
RECOVERY_SDCARD_ON_DATA
RECOVERY_TOUCHSCREEN_FLIP_X
RECOVERY_TOUCHSCREEN_FLIP_Y
RECOVERY_TOUCHSCREEN_SWAP_XY
TARGET_ARCH
TARGET_BOARD_PLATFORM
TARGET_BUILD_APPS
TARGET_BUILD_VARIANT
TARGET_CPU_VARIANT
TARGET_CUSTOM_KERNEL_HEADERS
TARGET_GLOBAL_CFLAGS
TARGET_HAS_BIGENDIAN
TARGET_HW_DISK_ENCRYPTION
TARGET_OUT_EXECUTABLES
TARGET_OUT_INTERMEDIATES
TARGET_OUT_OPTIONAL_EXECUTABLES
TARGET_OUT_SHARED_LIBRARIES
TARGET_OUT_VENDOR_SHARED_LIBRARIES
TARGET_PREBUILT_KERNEL
TARGET_PREFER_32_BIT
TARGET_RECOVERY_DEVICE_MODULES
TARGET_RECOVERY_FORCE_PIXEL_FORMAT
TARGET_RECOVERY_GUI
TARGET_RECOVERY_INITRC
TARGET_RECOVERY_OVERSCAN_PERCENT
TARGET_RECOVERY_PIXEL_FORMAT
TARGET_RECOVERY_QCOM_RTC_FIX
TARGET_RECOVERY_REBOOT_SRC
TARGET_RECOVERY_ROOT_OUT
TARGET_RECOVERY_UI_LIB
TARGET_RECOVERY_UPDATER_EXTRA_LIBS
TARGET_RECOVERY_UPDATER_LIBS
TARGET_ROOT_OUT
TARGET_ROOT_OUT_SBIN
TARGET_SIMULATOR
TARGET_USE_CUSTOM_LUN_FILE_PATH
TARGET_USERIMAGES_USE_EXT4
TARGET_USERIMAGES_USE_F2FS
TARGET_USES_LOGD
TW_ADDITIONAL_RES
TW_ALWAYS_RMRF
TW_BOARD_CUSTOM_GRAPHICS
TW_BRIGHTNESS_PATH
TW_CUSTOM_BATTERY_PATH
TW_CUSTOM_CPU_TEMP_PATH
TW_CUSTOM_POWER_BUTTON
TW_CUSTOM_THEME
TW_DEFAULT_BRIGHTNESS
TW_DEFAULT_LANGUAGE
TW_DISABLE_DOUBLE_BUFFERING
TW_DISABLE_TTF
TW_EXCLUDE_DEFAULT_USB_INIT
TW_EXCLUDE_ENCRYPTED_BACKUPS
TW_EXCLUDE_MTP
TW_EXCLUDE_SUPERSU
TW_EXTERNAL_STORAGE_MOUNT_POINT
TW_EXTERNAL_STORAGE_PATH
TW_EXTRA_LANGUAGES
TW_FBIOPAN
TW_FLASH_FROM_STORAGE
TW_FORCE_CPUINFO_FOR_DEVICE_ID
TW_HAS_DOWNLOAD_MODE
TW_HAS_NO_BOOT_PARTITION
TW_HAS_NO_RECOVERY_PARTITION
TW_HAVE_X86_ACCELERATED_PIXELFLINGER
TW_IGNORE_ABS_MT_TRACKING_ID
TW_IGNORE_MAJOR_AXIS_0
TW_IGNORE_MT_POSITION_0
TW_INCLUDE_BLOBPACK
TW_INCLUDE_CRYPTO
TW_INCLUDE_DUMLOCK
TW_INCLUDE_FB2PNG
TW_INCLUDE_INJECTTWRP
TW_INCLUDE_JB_CRYPTO
TW_INCLUDE_JPEG
TW_INCLUDE_L_CRYPTO
TW_INCLUDE_NTFS_3G
TW_INPUT_BLACKLIST
TW_INTERNAL_STORAGE_MOUNT_POINT
TW_INTERNAL_STORAGE_PATH
TW_MAX_BRIGHTNESS
TW_MTP_DEVICE
TW_NEVER_UNMOUNT_SYSTEM
TW_NEW_ION_HEAP
TW_NO_BATT_PERCENT
TW_NO_CPU_TEMP
TW_NO_EXFAT
TW_NO_EXFAT_FUSE
TW_NO_LEGACY_PROPS
TW_NO_REBOOT_BOOTLOADER
TW_NO_REBOOT_RECOVERY
TW_NO_SCREEN_BLANK
TW_NO_SCREEN_TIMEOUT
TW_NO_USB_STORAGE
TW_OEM_BUILD
TW_RECOVERY_ADDITIONAL_RELINK_FILES
TW_ROUND_SCREEN
TWRP_AUTOGEN
TWRP_CUSTOM_KEYBOARD
TWRP_EVENT_LOGGING
TWRP_FONTS_TTF
TWRP_INCLUDE_LOGCAT
TWRP_INCLUDE_XMLS
TWRP_NEW_THEME
TWRP_RES
TWRP_RES_GEN
TWRP_SH_TARGET
TWRP_THEME_LOC
TWRP_TOOLBOX_PATH
TW_SCREEN_BLANK_ON_BOOT
TW_SDEXT_NO_EXT4
TW_SECONDARY_BRIGHTNESS_PATH
TW_TARGET_USES_QCOM_BSP
TW_THEME
TW_USE_KEY_CODE_TOUCH_SYNC
TW_USE_MODEL_HARDWARE_ID_FOR_DEVICE_ID
TW_USE_TOOLBOX
TW_WHITELIST_INPUT
TW_X_OFFSET
TW_Y_OFFSET

PS we should have bigbiff provide descriptions for every one of them! (partially kidding)