configs/avenger96_defconfig: add support for Arrow Avenger96 board

Very similar to the other stm32mp157-based boards, except that we use the
multi_v7 defconfig for ease of maintenance.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/arrow/avenger96/overlay/boot/extlinux/extlinux.conf

@@ -0,0 +1,4 @@
+label stm32mp157c-dk2-buildroot
+  kernel /boot/zImage
+  devicetree /boot/stm32mp157a-dhcor-avenger96.dtb
+  append root=/dev/mmcblk1p4 rootwait

board/arrow/avenger96/readme.txt

@@ -0,0 +1,41 @@
+Arrow Avenger96
+
+Intro
+=====
+
+This configuration supports the Arrow Avenger96 board:
+
+https://wiki.dh-electronics.com/index.php/Avenger96
+
+How to build
+============
+
+ $ make avenger96_defconfig
+ $ make
+
+How to write the microSD card
+=============================
+
+WARNING! This will destroy all the card content. Use with care!
+
+Once the build process is finished you will have an image called
+"sdcard.img" in the output/images/ directory.
+
+Copy the bootable "sdcard.img" onto an microSD card with "dd":
+
+  $ sudo dd if=output/images/sdcard.img of=/dev/sdX
+
+Boot the board
+==============
+
+ (1) Configure the boot switches for boot from microsd: 1-0-1
+
+ (2) Insert the microSD card in the slot
+
+ (3) Plug a serial adapter (beware: 1v8 levels!) to the low speed
+     expansion connector
+
+ (4) Plug in power cable
+
+ (5) The system will start, with the console on UART, but also visible
+     on the screen.