Make sure that DTB files are included on boot partition.

This is important so that the bootloader (U-Boot) can use the DTB
files when chainloading GRUB via UEFI. The logic for doing this is
already present in `distro_bootcmd`, but because we were not placing
the DTB files in the correct folder, this part was skipped.

Changelog: Fix certain kernels hanging on boot. In particular, recent
versions of Debian for Beaglebone was affected, but several other
boards using UEFI may also have been affected.

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>

mender-convert-modify

@@ -164,6 +164,18 @@ EOF
 
   run_and_log_cmd "sudo mkdir -p work/boot/EFI/BOOT"
   run_and_log_cmd "sudo cp work/${efi_name} -P work/boot/EFI/BOOT/${efi_target_name}"
+
+  # Copy dtb directory to the boot partition for use by the bootloader.
+  if [ -d work/rootfs/boot/dtbs ]; then
+    # Look for the first directory that has dtb files. First check the base
+    # folder, then any subfolders in versioned order.
+    for candidate in work/rootfs/boot/dtbs $(find work/rootfs/boot/dtbs/ -maxdepth 1 -type d | sort -V -r); do
+      if [ $(find $candidate -maxdepth 1 -name '*.dtb' | wc -l) -gt 0 ]; then
+        run_and_log_cmd "cp -r $candidate work/boot/dtb"
+        break
+      fi
+    done
+  fi
 fi
 
 run_and_log_cmd "sudo mkdir -p work/rootfs/data/mender"