cmake: modules: boards: Add normalised variables

Adds a new variable NORMALIZED_BOARD_QUALIFIERS which contains
the board qualifiers in file-name format, this allows for
constructing strings in applications (e.g. for folder names) prior
to Zephyr being found for things like APPLICATION_CONFIG_DIR. Also
adds NORMALIZED_BOARD_TARGET.

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