mpc85xx: Fix output location of padded dtb

mpc85xx: Fix output location of padded dtb

In commit 7e614820a892 ("mpc85xx: add support for Extreme Networks
WS-AP3825i"), we borrowed a recipe convention from apm821xx for device
tree blob padding. Unfortunately, in the apm821xx target, the image
recipes name the device tree blob differently, meaning that in
mpc85xx, the padded dtb is never consumed.

Change the definition of `Build/dtb` so that it outputs the padded dtb
to the correct location for it to be consumed.

Also, rename the recipe to `Build/pad-dtb`, so it is clear we
are building and padding the device tree blob.

This change fixes Github issue #9779 [1].

[1]: https://github.com/openwrt/openwrt/issues/9779

Fixes: 7e614820a892 ("mpc85xx: add support for Extreme Networks WS-AP3825i")
Signed-off-by: Martin Kennedy <hurricos@gmail.com>
(cherry picked from commit 1d06277407d3e294589ecde46328303a9f3803fd)