kernel-devicetree: recursively search for dtbs

kernel-devicetree: recursively search for dtbs

Upstream's dtb directory structure has no real standard. They just tend
to idle around the 2/3 directory depth. Recursively search for the
dtb/dtbo files instead of assuming anything.

Fixes: 04ab57d200 (kernel-devicetree: allow specification of dtb
directory, 2023-05-02)

Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>