kbuild: Simplify generating vmlinux

Generating vmlinux in top-level Makefile were getting a bit messy after kallsyms
support were added. Also the full link of all the .o files were duplicaed a number of times.
This patch does the following:
- Introduce built-in.o which is a prelink of most .o files
- Make the build process a bit more verbose telling when linking .tmpvmlinux*
- Use less magic when determing when to generate a new version
- Allow architectures to override the defineition of cmd_vmlinux__
- Add more comments to the MAkefile and clean up soem other comments
- Display more commends during V=1 builds

The resulting kernel boots and rn here.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>