Finally I managed to build an RT_PREEMPT kernel.
in meta-clanton_v0.7.5/meta-clanton-bsp/recipes-kernel/linux/linux-yocto-clanton_3.8.bb
1) replace standard kernel with rt-patched kernel source
SRC_URI = "git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git;branch=linux-3.8.y"
SRCREV = "531ec28f9f26f78797124b9efcf2138b89794a1e"
with
SRC_URI = "git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git;branch=v3.8-rt"
SRCREV = "1ceec8c6737b01eeebdf1b40c3da7e489b7f02d5"
2) adapt the clanton.patch to fit the new kernel, mainly code-adaptations and some asm-generic/ includes to change in asm/
3) take the new kernel .config and add to it the clanton.cfg special CONFIG's (run a diff to find and check the differences)
4) run bitbake linux-yocto-clanton
5) go to yocto_build/tmp/work/clanton-poky-linux-uclibc/linux-yocto-clanton/3.8-r0/linux and run patch -p1 < ../clanton.patch
6) replace the .config of yocto_build/tmp/work/clanton-poky-linux-uclibc/linux-yocto-clanton/3.8-r0/linux-clanton-standard-build with the clanton.cfg renamed .config
7) run bitbake linux-yocto-clanton -c compile
8) run bitbake image-full (or image-spi)
the result image has rt-patched kernel. This is not the most beautiful solution (e.g. the label "linux-yocto-standard" is no more correct) but it seems to work. Some tests should be done.