When performing non standalone builds a separate autoconf.h file is generated and defines
the configuration of the kernel. Currently the directory with that file as well as the
kernels include/plat/${PLAT} directory are both listed as include directories to the
compiler. Due to the directory with the generated autoconf.h being earlier in the -I
list the generated autoconf.h will be used in preference to the standalone autoconf.h.
Relying on the order of -I directives is too unstable and so this commit moves the
autoconf.h files to a dedicated configs directory, which is only added as a -I
directive if a standalone build is being performed.
