Make folder for dtb files as an argument

Some distribution use different folders to store the dtb files
rather then storing them directly in /boot. This adds a new
argument to define in which folder U-Boot should search the dtb
file.

Signed-off-by: Stefan Agner <stefan@agner.ch>
[swarren, converted trailing / test to use .endswith()]
Signed-off-by: Stephen Warren <swarren@nvidia.com>