Fix GRUB2 update using post-install hook (#1770)

* Use shell functions for install hooks

* Use post-install hook to initialize GRUB2 bootloader env

Unfortunately the boot name to be updated (RAUC_SLOT_BOOTNAME) is not
available when updating the "boot" slot. Instead, initialize the boot
slot in a kernel post-install slot.

buildroot-external/scripts/ota.sh

@@ -38,6 +38,9 @@ function create_ota_update() {
         echo "hooks=install"
         echo "[image.kernel]"
         echo "filename=kernel.img"
+        if [ "${BOOTLOADER}" == "grub" ]; then
+            echo "hooks=post-install"
+        fi
         echo "[image.rootfs]"
         echo "filename=rootfs.img"
     ) > "${rauc_folder}/manifest.raucm"