kbuild: Update Documentation/kbuild/makefiles.txt

makefiles.txt brought up-to-date with the changes that has occured
in kbuild within the last couple of months.
Restructured to present relevant info earlier, and rewritten the
architecture specific section to a certain degree.

One change in style is that makefiles used throughout the kernel tree is called
"kbuild makefiles", because they follow the kbuild syntax.
Old notation was "subdirectory makefiles".

There is added a TODO section, if anyone feel tempted to add a bit more text.