coreboot: Add kevin chromebook configs

This adds coreboot configuration for the Samsung Chromebook Plus (v1),
which is based on the "google/gru" mainboard in upstream coreboot. Also
adds a shared "gru" 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=0x00800000
    CONFIG_BOARD_GOOGLE_KEVIN=y
    CONFIG_CONSOLE_CBMEM_BUFFER_SIZE=0x20000
    CONFIG_UART_PCI_ADDR=0x0
    CONFIG_I2C_TRANSFER_TIMEOUT_US=500000
    CONFIG_PAYLOAD_FIT_SUPPORT=y

Most things work, but one significant problem is that the board can't power
off properly. It also happens with my manual U-Boot-only builds, but not
when I manually build coreboot with a U-Boot payload. Not sure why it is
happening here as well.

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

resources/coreboot/gru/board.cfg

+cbtree="gru"
+cbrevision="8da4bfe5b573f395057fbfb5a9d99b376e25c2a4" # 4.17
+romtype="normal"
+arch="AArch64"

resources/coreboot/gru_kevin/board.cfg

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