From: Sam Ravnborg <sam@ravnborg.org>

When utilising the make O=...  option the include options for gcc were
mangled even when absolute paths was used.  Also remove duplication of
CPPFLAGS.  They were assigned twice.  [It is still possible for
architectures to modify CPPFLAGS].

This patch allows xconfig to be build with make O=...  xconfig.It will also
help development of external modules with absolute paths for their -I
options.

Note: As a side effect a full recompile of the kernel takes place due to
changes in number of gcc options.