coreboot: Add bob chromebook configs

This adds coreboot configuration for the ASUS Chromebook Flip C101,
which is based on the "google/gru" mainboard in upstream coreboot. It
uses the shared tree for the "gru" baseboard.

The config is based on the following defconfig:

    # CONFIG_USE_BLOBS is not set
    CONFIG_VENDOR_GOOGLE=y
    CONFIG_CBFS_SIZE=0x00800000
    CONFIG_BOARD_GOOGLE_BOB=y
    CONFIG_DRIVER_TPM_SPI_BUS=0x0
    CONFIG_CONSOLE_CBMEM_BUFFER_SIZE=0x20000
    CONFIG_UART_PCI_ADDR=0x0
    CONFIG_I2C_TRANSFER_TIMEOUT_US=500000
    CONFIG_PAYLOAD_FIT_SUPPORT=y

Untested since I don't have the bob chromebook.

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

resources/coreboot/gru_bob/board.cfg

+cbtree="gru"
+romtype="normal"
+arch="AArch64"
+payload_uboot="y"