Just temporarily make a new rpmbuild folder elsewhere by redefining "%_topdir" in ~/.rpmmacros. For example:
You'll need to create the regular directories before getting started:
cd /a/new/location/rpmbuild; mkdir -p ./BUILD ./SPECS ./SRPMS ./BUILDROOT ./RPMS/i386 ./RPMS/i686 ./RPMS/x86_64 ./RPMS/athlon ./RPMS/i486 ./RPMS/i586 ./RPMS/noarch ./SOURCES
Now reinstall the kernel SRPM and you should be able to build using the .spec file in /a/new/location/rpmbuild/SPECS/kernel.spec.