Enhancement: option to reduce TLV re-reads

[de-nordic]

What is a problem?

We keep on re-reading TLV in various places in MCUboot, which usually means that we try to access flash, and iterate over some are of it, several times.
We could probably have an option to cache the TLV read and keep it in memory to reduce on need to access storage.

What are the costs?

Some additional logic for read and additional memory for caching.

What do we gain?

Reduce need to access flash every time so improved boot time and reduced power consumption during boot phase, especially when it is needed to read TLV from externally connected device.