You can still say that there is a Raspberry-Pi inside, just like you can say that there is an ARM inside.
Don't use the logo or in any other way suggest (let alone say) that you are related or approved by them.
Section 4.II clearly states:
Your use of the Raspberry Pi Marks must also comply with the following rules:
(i) Any use of the Raspberry Pi Marks must be accompanied by the acknowledgement “Raspberry Pi is a trademark of the Raspberry Pi Foundation”.
(ii) Where you are using the Raspberry Pi Word Mark to state or indicate that another product is compatible with, works with or can be used with our products, you must use it with a referential phrase such as “runs on,” “for use with,” “for,” or “compatible with”.
So you can say "contains".
I suggest that if you make modifications to your PI you also clearly states that, just in case your modifications cause trouble which may produce a negative image (Also in the Trademark rules)