Fix incorrect mapping of DT overlays on RPi 5 (#3335)

RPi 5 config uses BR2_LINUX_KERNEL_INSTALL_INTREE_OVERLAYS which builds
the device tree overlays from the Linux tree when building the kernel.
The overlays directory also contains overlay_map.dtb which is necessary
to correctly map overlays without RPi version suffix to the
platform-specific ones. Without this, some peripherals may not work
correctly on Pi 5 without any obvious error messages in the kernel log
because an incorrect (Pi 4) overlay is used as the default.

Fixes #3321

* buildroot cc0481f40e...0a64bfe8f1 (1):
  > Install overlay_map.dtb when using in-tree DT overlays