Enhancement: cache TLVs instead of re-reading them from flash

[de-nordic]

** Problem description **
TLVs are accessed and processes multiple times while validating, swapping and processing images. This is waste of flash access time and requires processing of possible read problems each time.

** Proposed solution **
Read all TLVs to RAM and iterate over them in RAM rather than flash, every time access to TLVs is required.