Remove unnecessary GRUB userspace tools and other files (#3887)

Because the OTA hooks interact with GRUB environment using grub-editenv, we
have BR2_TARGET_GRUB2_INSTALL_TOOLS enabled. However, that brings massive bloat
of files that are never used in HAOS, as it also installs many other binaries,
GRUB modules and translations.

As it's not possible to configure what gets installed in grub2 package, remove
the undesired files in the post-build function. This brings savings of ~8.5MB
of space in the root partition.

buildroot-external/scripts/rootfs-layer.sh

@@ -26,6 +26,15 @@ function fix_rootfs() {
 
     # Use systemd-resolved for Host OS resolve
     sed -i '/^hosts:/ {/resolve/! s/files/resolve [!UNAVAIL=return] files/}' "${TARGET_DIR}/etc/nsswitch.conf"
+
+    # Remove unnecessary grub userspace tools, config, modules and translations
+    find "${TARGET_DIR}"/usr/{,s}bin -name "grub-*" -not -name "grub-editenv" -delete
+    rm -rf "${TARGET_DIR}/etc/grub.d"
+    rm -rf "${TARGET_DIR}/usr/lib/grub"
+    if [ -d "${TARGET_DIR}/share/locale" ]; then
+        find "${TARGET_DIR}/share/locale" -name "grub.mo" -delete
+        find "${TARGET_DIR}/share/locale" -type d -empty -delete
+    fi
 }