uboot-sign: Fix potential index error issues

Someone reported that if some other shell function has left i or j set,
the concat_dtb_helper function could fail. Add a small tweak to avoid this.

[YOCTO #14815]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>