zephyr: restore device tree overlay

0e3fa72d removed device tree overlay
that is required to flash mcuboot to correct partition and CMake
variable that pointed to it. Restore it.

Do not restore adding "boards/${BOARD}.overlay" to the list since Zephyr
detects this file automatically now - see https://docs.zephyrproject.org/latest/application/index.html#devicetree-overlays



Signed-off-by: Sergey Koziakov <dya.eshshmai@gmail.com>