Cleanup existing `<normalized_board_target>.yml` files used by twister and merge content into `board.yml`

[tejlmand]

During introduction of Hardware Model v2 the current <normalized_board_target>.yml file was discussed.

To minimize the number of various yaml files in board folders, then the content of board.yml and <normalized_board_target>.yml should be merged.

This will also remove the need for duplicated content in both files.

Before unifying the files, then the content of <normalized_board_target>.yml must be cleaned up.

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See also: #50305 (comment)
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[kylebonnici]

in this yaml file a field for name was available and this was very useful to give a board some friendly name. It would be greatly appreciate when the <normalized_board_target>.yaml is merged into the board.yml the name is also merged. I would propose that this field is renamed to 'friendlyName'.

Example:
board:
name: nrf5340dk
friendlyName: 'Nordic nRF5340DK board'
....
....

I guess that this field should be optional.