zephyr: fix handling of devicetree overlays in mcuboot

The current specific setting of devicetree overlay files using `set()`
has a couple of built-in flaws.

It keeps readding the overlay file on each subsequent CMake invocation.
The build command (make/ninja), will automatically invoke CMake if there
are any changes to files used as configure time dependencies.

This can easily be seen by manually re-invoking CMake:
```
 # First invocation
$ cmake -DBOARD=nrf52840dk_nrf52840  -DDTC_OVERLAY_FILE=custom.dts ..
Loading Zephyr default modules (Zephyr workspace).
-- Application: /projects/github/ncs/bootloader/mcuboot/boot/zephyr
...
-- Found devicetree overlay: custom.dts
-- Found devicetree overlay: bootloader/mcuboot/boot/zephyr/dts.overlay

 # Second invocation
$ cmake -DBOARD=nrf52840dk_nrf52840  -DDTC_OVERLAY_FILE=custom.dts ..
Loading Zephyr default modules (Zephyr workspace).
-- Application: /projects/github/ncs/bootloader/mcuboot/boot/zephyr
...
-- Found devicetree overlay: custom.dts
-- Found devicetree overlay: mcuboot/boot/zephyr/dts.overlay
-- Found devicetree overlay: mcuboot/boot/zephyr/dts.overlay
```

Zephyr has built-in support for application specific overlay config
which gets automatically applied when the overlay file is named:
`app.overlay`.

Therefore rename `dts.overlay` to `app.overlay`.
Ref: https://docs.zephyrproject.org/3.2.0/build/dts/howtos.html

 \
  #set-devicetree-overlays

This change further allows users of mcuboot to place their mcuboot
configuration out-of-tree of the sample by using the
`APPLICATION_CONFIG_DIR` setting.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>