Remove Flash Partition description from Board definition and into Application overlays

[cowsed]

Describe the bug
Historically, we've only had one program per board. So, we defined our flash partitions for that one program in the board.dts file . While this has always been wrong, its only recently become an issue. As we start to reuse boards for multiple things, other applications other than the 'main' one that the board was originally used for have to use hacky and weird delete-node commands in their boards/BOARD.overlay file

Expected behavior
Partitions specific to an application should go in that apps boards/BOARD_NAME.overlay file
Both flash fixed_partitions and littlefs partition definition