Normative/informative references

[jiceatscion]

Make sure the references are righttly placed according to guidelines:

• Normative references: Needed for the specification; specification is based on it.
• Informative references: Provide background/additional information

Potential changes:

• [x] I-D.scion-cppki should be normative
• [x] Several RFCs might be informative
• [x] Check if other drafts affected

[nicorusti]

@nicorusti double check before resolving