Fix MIPS BR2_PACKAGE_LINUX_TOOLS_GPIO build

Update mips for snapshot and initrd features.

Document MIPS limitations.

Improve description of test procedure

run

@@ -116,12 +116,15 @@ $extra_flags \
 "
   ;;
   mips64)
+    if ! $initrd; then
+      root='root=/dev/hda'
+      extra_flags="$extra_flags -drive file='${images_dir}/rootfs.ext2.qcow2,format=qcow2'"
+    fi
     cmd="$qemu_common \
 -M malta \
 -append 'root=/dev/hda $extra_append' \
 -cpu I6400 \
 -device pcnet \
--drive file=${images_dir}/rootfs.ext2,format=raw \
 -kernel ${images_dir}/vmlinux \
 -nographic \
 $extra_flags \