Make examples not board specific

[Ivan-Velickovic]

Most of the examples are just copy-pastes of the 'hello world' example for each board which doesn't actually need to be board specific.

Nowadays there are enough boards that it's becoming annoying to deal with so we should change the file structure to something like:
examples/hello/*

and have it work for every board by passing MICROKIT_BOARD=<board>.

And for the actually platform specific examples such as example/odroidc4/timer, we should change it to something like:
example/timer/board/odroidc4/timer.system and all the other non-platform specific files go in example/timer/.