cd /root/rpmbuild/BUILD/kernel-2.6.18/xen/arch/x86
cp configs/<desired-config-file> .config
make oldconfig
make menuconfig
ou
make xconfig
Então
cp .config ~/rpmbuild/SOURCES/config-'uname -m'-generic
cd /root/rpmbuild
rpmbuild -bb --target='uname -m' kernel.spec