Loading different DTS files from U-Boot

We're at a point where it probably makes sense to separate the generic
"hey, let's experiment with this" Clearfog config from a device-specific
"I'm just a CL-ROADMv2" or "I'm a future SDN ROADM thingy".

The CL-ROADMv2 is branched out in order to cement this legacy, one-off
device in stone. Note that ADT7643 is still being loaded in a crazy way
as explained by https://gerrit.cesnet.cz/483 .

Change-Id: Ie86d37cf94c36077134afa54cacdae6962dd8c11
diff --git a/board/czechlight/clearfog/boot.scr.txt b/board/czechlight/clearfog/boot.scr.txt
index cf669ea..2777ba8 100644
--- a/board/czechlight/clearfog/boot.scr.txt
+++ b/board/czechlight/clearfog/boot.scr.txt
@@ -1,4 +1,8 @@
 setenv bootargs root=/dev/mmcblk0p${rauc_part} rauc.slot=${rauc_slot} czechlight=${czechlight} systemd.machine_id=${machineid} panic=10 oops=panic
-load mmc 0:${rauc_part} ${fdt_addr_r} /boot/czechlight-clearfog.dtb || reset
+if load mmc 0:${rauc_part} ${fdt_addr_r} /boot/${czechlight}-clearfog.dtb; then
+  echo "DTS: ${czechlight}-clearfog"
+else
+  load mmc 0:${rauc_part} ${fdt_addr_r} /boot/czechlight-clearfog.dtb || reset
+fi
 load mmc 0:${rauc_part} ${kernel_addr_r} /boot/zImage || reset
 bootz ${kernel_addr_r} - ${fdt_addr_r} || reset