rdtech-tc: rephrase PAC consistency check, discuss sample data layout

rdtech-tc: rephrase PAC consistency check, discuss sample data layout

Rephrase access to the PAC chunks and the consistency check of the total
192 bytes data image in terms of one 64 bytes unit. Which eliminates
other magic numbers. Add a comment for awareness during development.

Prefer read_u32le() endianess conversion helpers for reliability. Use
big endian readers for four-character-codes to increase readability.
Address style nits in the process: Use booleans for checksum and
consistency checks. Eliminate else after return. Consolidate error
messages. Prefer indexing an array over pointer arithmetics. Group
related instructions in the same block.