Automatically generated linker scripts lack CMake integration

[cwedgwood]

There are several linker scripts which are generated from LUA scripts.

In general it would be good to teach the build system that in some cases mos-platform/target/something.ld depends on mos-platform/target/misc/generate-something.lua.

It doesn't have to be a uniform pattern, we can add an explicit dependency or pattern for each target.

[asiekierka]

Lua would add a new dependency to the build system - we've had a discussion about this ( #205 ) and decided we'd like to set a standard for such script-based linker script generation first, then integrate it with the build system second.