configs/olimex_a20_olinuxino_micro: bump Linux to 6.18.8 and U-Boot to 2026.01

The patch:
* bumps Linux kernel to version 6.18.8
* bumps U-Boot to version 2026.01
* forces check hashes
* switches to extlinux

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[Julien: change linux.hash comment to use hash from upstream]
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/olimex/a20_olinuxino/boot.cmd

@@ -1,5 +0,0 @@
-setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p1 rootwait panic=10 ${extra}
-ext4load mmc 0 0x49000000 /boot/${fdtfile}
-ext4load mmc 0 0x46000000 /boot/zImage
-env set fdt_high ffffffff
-bootz 0x46000000 - 0x49000000

board/olimex/a20_olinuxino/patches/linux/linux.hash

@@ -1,2 +1,3 @@
-# Locally computed
+# From https://www.kernel.org/pub/linux/kernel/v6.x/sha256sums.asc
 sha256  7a8879167b89c4bae077d6f39c4f2130769f05dbdad2aad914adab9afb7d7f9a  linux-6.18.3.tar.xz
+sha256  37f0c5d5c242c1d604e87d48f08795e861a5a85f725b4ca11d0a538f12ff8cff  linux-6.18.8.tar.xz

board/olimex/a20_olinuxino/post-build.sh

@@ -1,3 +0,0 @@
-#!/bin/sh
-
-cp $BINARIES_DIR/boot.scr $TARGET_DIR/boot/boot.scr

board/olimex/a20_olinuxino/rootfs_overlay/boot/extlinux/extlinux.conf

@@ -0,0 +1,4 @@
+label A20-Olinuxino
+    kernel /boot/zImage
+    fdtdir /boot
+    append console=ttyS0,115200 root=/dev/mmcblk0p1 rootwait panic=10 ${extra}