devkit: Make dtb copy unambiguous

Using uname -r in the cp source.

Development_Environment/Boards/imx8.rst

@@ -274,7 +274,7 @@ Put the `librem5-evk-lcdonly.dtb` file in the correct place:
 
 ::
 
-   root@pureos:~# cp /usr/lib/linux-image-4.18.11-*/freescale/librem5-evk-lcdonly.dtb /boot/dtbs
+   root@pureos:~# cp /usr/lib/linux-image-`uname -r`/freescale/librem5-evk-lcdonly.dtb /boot/dtbs
    root@pureos:~# cp /boot/dtbs/librem5-evk.dtb /boot/dtbs/librem5-evk.dtb.bak
    root@pureos:~# cp /boot/dtbs/librem5-evk-lcdonly.dtb /boot/dtbs/librem5-evk.dtb