rootfs_postbuild: Run rauc_fixups here instead of in the makefile #5

This is more consistent and allows updating the certs if needed.

I discussed this with Xogium and she pointed out that there's still a race between postbuild and genimage when it comes to reading the certificates. Also that it's bad to use postbuild for this type of fixing.

So I've redone the commits to:

• Use 'world' instead of 'all' target so rauc-fixups in make works properly
• Copy certificates and private keys to the build directory so there's no race conditions with external modification

rootfs_postbuild: Run rauc_fixups here instead of in the makefile