Wedge400 EXT4 kernel option not enabled

The BSM eMMC is configured to be formatted and mounted as EXT4. But no kernel support is available to mount. 

Adding the following to ./meta-facebook/meta-wedge400/recipes-kernel/linux/board_config/wedge400.cfg fixes this issue.
CONFIG_EXT4_FS=y
CONFIG_EXT4_USE_FOR_EXT2=y