Support for Embedded Plain AES keys

[de-nordic]

Support in code for querying user provided function for an AES encryption key.

Extra Zephyr code and template that allows to use the new functionality.

Although I think this is ready for review I am still working, in different set of commits, on redefining encryption key type so that we could abstract that type across the MCUboot code; this means that this PR may change in how boot_take_enc_key function is defined.

~~Update: I will move imgtool script to separate PR, at this point. Fighting with FIH tests that from, what seems to be a timeout, without any feedback is impossible. Running personal docker is impossible either, because after getting authorized with docker registry I am denied access to required data anyway.~~ Finally fixed the scripts to make FIH and TrustedFirmware-M happy.