Move osflow examples

[stnolting]

This PR moves the board tops used by the open-source flows from setups/examples to osflow/board_top.

The original setups/example folder was meant as a board-wrappers folder that can be used by any toolchain (for example by Lattice Radiant). But I think it is cleaner and easier to understand if we have these board-tops inside the according toolchain folders even if there might be some redundancy (for example is ghdl-yosys and Radiant could use the same board tops).

Another thing is that Radiant and ghdl-yosys seem to expect different primitive libraries:

neorv32/setups/examples/neorv32_UPduino_BoardTop_UP5KDemo.vhd

Lines 39 to 40 in 70edf18

	library iCE40;
	use iCE40.components.all; -- for device primitives and macros

vs.

neorv32/setups/radiant/UPduino_v3/neorv32_upduino_v3_top.vhd

Lines 45 to 46 in 70edf18

	library iCE40UP;
	use iCE40UP.components.all; -- for device primitives

[stnolting]

In theory this is ready to merge.

edit

But I think it would be good to do another release of the current setup before merging. Furthermore, we should wait until #139 is merged 😉