Well after some attempt, i've been able to get a read only rootfs using st-image-core ans SDK2.
I've found that if I modify local.conf by adding "read-only-rootfs" in EXTRA_IMAGE_FEATURES
it cannot bitbake , stating errors about vendorfs and ldconfig
But, if i made a copy of st-image-core.bb (and rename it) and put it in my own layer (/meta-something/images/my-custom-image.bb)
i add 'read-only-rootfs' in the "image-features-part"
It bitbakes ok, and i have a rootfs in readonly mode.
The final goal is to have an almost stateless device in order to have a robust embedded device.
Next step, dive into simlink and others things!
seeya!
links that helped, in french, sorry!
https://www.blaess.fr/christophe/yocto-lab/sequence-II-2/index.html#utilisation-des-fonctionnalites-d-image
https://www.blaess.fr/christophe/yocto-lab/sequence-III-1/index.html#systeme-de-fichiers-en-lecture-seule