A platform in this context is a set of different boards that are similar, e.g. PC motherboards are different, but together they form a platform that can be treated uniformly. A SoC is a System on a Chip, i.e. a CPU and associated peripheral circuits are integrated on one silicon chip. The same SoC can be used on different boards designs, in which case they ...
It looks like Petalinux uses /etc/rcS.d for its init scripts, the following is a step by step that should allow you to resolve this issue:
Please let us know if you have any further issues.