coreboot: Add nyan big chromebook configs

This adds coreboot configuration for the Acer Chromebook 13 (CB5-311,
C810), which is based on the "google/nyan_big" mainboard in upstream
coreboot. Also adds a shared "nyan" board directory to share with
others having the same baseboard.

The config is based on the following defconfig:

    # CONFIG_USE_BLOBS is not set
    CONFIG_VENDOR_GOOGLE=y
    CONFIG_CBFS_SIZE=0x400000
    CONFIG_BOOT_DEVICE_SPI_FLASH_BUS=4
    CONFIG_BOARD_GOOGLE_NYAN_BIG=y
    CONFIG_CONSOLE_CBMEM_BUFFER_SIZE=0x20000
    CONFIG_DRIVERS_AS3722_RTC_BUS=4
    CONFIG_DRIVERS_AS3722_RTC_ADDR=0x40
    CONFIG_UART_PCI_ADDR=0x0
    CONFIG_I2C_TRANSFER_TIMEOUT_US=500000

Untested since I don't have the nyan big chromebook.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>