Lets users over-ride with makeoptions COPTS="..." in kernel config files.
Leave `mandatory' flags (like -msoft-float which on m68k enforces no
FP in kernel) in CFLAGS.
* Make S expand to an absolute path at compile time.
* Use `-S' rather than `-x' to remove debugging symbols.
* Garbage collect unused variables.
* Reverse a handful of port-specific changes that do not correspond to
the common build model and are not needed.