board: Add Bananapi M1 support

Add initial support for bananapi M1 board with below features:
- U-Boot 2017.09
- Linux 4.13.7
- Default packages from buildroot

Cc: Jason <manager@sinovoip.com.cn>
Cc: hailymei@banana-pi.com <hailymei@banana-pi.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

configs/bananapi_m1_defconfig

@@ -0,0 +1,37 @@
+BR2_arm=y
+BR2_cortex_a7=y
+BR2_ARM_EABIHF=y
+
+# Linux headers same as kernel, a 4.13 series
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_13=y
+
+# Bootloader
+BR2_TARGET_UBOOT=y
+BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
+BR2_TARGET_UBOOT_CUSTOM_VERSION=y
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2017.09"
+BR2_TARGET_UBOOT_BOARD_DEFCONFIG="Bananapi"
+BR2_TARGET_UBOOT_NEEDS_DTC=y
+BR2_TARGET_UBOOT_FORMAT_BIN=y
+BR2_TARGET_UBOOT_SPL=y
+BR2_TARGET_UBOOT_SPL_NAME="u-boot-sunxi-with-spl.bin"
+BR2_TARGET_UBOOT_BOOT_SCRIPT=y
+BR2_TARGET_UBOOT_BOOT_SCRIPT_SOURCE="board/bananapi/bananapi-m1/boot.cmd"
+
+# Kernel
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.13.7"
+BR2_LINUX_KERNEL_DEFCONFIG="sunxi"
+BR2_LINUX_KERNEL_DTS_SUPPORT=y
+BR2_LINUX_KERNEL_INTREE_DTS_NAME="sun7i-a20-bananapi"
+
+# Filesystem
+BR2_TARGET_GENERIC_ISSUE="Welcome to Bananapi M1"
+BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_EXT2_4=y
+BR2_PACKAGE_HOST_DOSFSTOOLS=y
+BR2_PACKAGE_HOST_GENIMAGE=y
+BR2_PACKAGE_HOST_MTOOLS=y
+BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"
+BR2_ROOTFS_POST_SCRIPT_ARGS="-c board/bananapi/bananapi-m1/genimage.cfg"