Add a step to derive microdroid vendor dice node

The derivation happens in the derive_microdroid_vendor_dice_node binary
which first_stage_init forks and execvs.

Since the derivation requires talking to the dice driver, its
initialisation is also moved to the first stage init.

The derivation happens before the microdroid vendor partition is
verified & mounted. This should be safe because the first_stage_init
will fail the boot if the verification of the microdroid vendor
partition fails.

Bug: 287593065
Test: run microdroid with and without vendor partition
Test: atest MicrodroidTests
Change-Id: I0d83772eb98a56c315617e66ec64bd03639cfde6