EifReader::from_eif does not read sections according to metadata in EifHeader

[foersleo]

The EifReader::from_eif function does not honor the description of sections set in the EifHeader. Instead, it starts parsing section headers starting directly after the last element it has parsed using a file seek based scheme (https://github.com/aws/aws-nitro-enclaves-image-format/blob/main/src/utils/eif_reader.rs#L103-L107). This means it does not do the validation of the file in the same way as the Nitro virtualization stack which goes by the section boundaries described in EifHeader.

This can be problematic with hand-crafted EIF that contains more data than the sections (e.g. gaps with random data between section). Handling of such files might yield undefined nitro-cli behavior.

[maayank]

Also addressed in PR #29