cmake: modules: extensions: Revert using common board files

This change reverts a change that was introduced with hwmv2 which
allowed for using common board names for overlays, given the board
target ``nrf9160dk@0.7.0/nrf9160/ns``.

In hwmv1 this would have used:
    ``nrf9160dk_nrf9160_ns.conf``.
    ``nrf9160dk_nrf9160_ns_0_7_0.conf``.

In hwmv2 this would have used:
    ``nrf9160dk_nrf9160_ns.conf``
    ``nrf9160dk_nrf9160_ns_0_7_0.conf``
    ``nrf9160dk_nrf9160.conf``
    ``nrf9160dk_nrf9160_0_7_0.conf``
    ``nrf9160dk.conf``
    ``nrf9160dk_0_7_0.conf``

With these changes, the following are used (which restores the hwmv1
behaviour):
    ``nrf9160dk_nrf9160_ns.conf``
    ``nrf9160dk_nrf9160_ns_0_7_0.conf``

For a board with a default SoC which is not a variant for example
``rpi_pico`` then ``rpi_pico.conf`` will also be used, this file
will not be used for variants e.g. ``rpi_pico/rp2040/w``

This applies to .dts, .conf and .overlay files in the boards
directory, and to .conf and .overlay files in application board
overlay directories.

This revert is needed to avoid issues whereby variants have
incompatible configuration to the parent board target, which has
been affecting samples and tests.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>