Incremental boot image compilation

Currently the boot image writer performs the task of finding dependencies in code, compiling it, linking the code and data into the boot image files (including generating the an initial root set of boot image objects). A performance improvement allows the compilation to occur in parallel.

The approach of the boot image writer means that changing one source file requires the entire boot image to be rebuilt. If we factored out the different jobs of the boot image writer and used intermediate files, we could create a boot image compiler and boot image linker. Such a system should enable us to recompile only modified files into the boot image.

This issue was previously SF RFE #1643564.