Add ADR about not using FDE on Pis

doc/adr/0002-no-fde-on-unattended-devices.md

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+# 2. No FDE on Unattended Devices
+
+Date: 2022-07011
+
+## Status
+
+Proposed
+
+## Context
+
+One of the core concepts of using Raspberry Pi devices as sensors is that the
+sensors may be physically located in locations that inhibit, make inconvenient,
+or even prevent direct access (e.g., mounted on top of lamp posts in library
+parking lots).  As a result, these devices, for all intents and purposes, are
+unattended.
+
+Encryption -- including Full Disk Encryption (FDE) -- requires a key of some
+sort to unencrypt the filesystem(s) stored on the device.  This key may be
+stored on a removable storage device (e.g., USB drive) or it may be typed
+by an operator at system boot.  This prevents unauthorized access should the
+physical media backing the system become compromised.  If the key to unencrypt
+the media is present on the device (e.g., stored as a file), the storage
+may as well not be encrypted at all.
+
+As a result, using Full Disk Encrpytion (FDE) on a device required an
+operator to provide the key in order for the system to boot; without an
+operator providing the key, the system will not boot.  Therefore, devices
+must either be attended or they may not utilize Full Disk Encryption.  Since
+the devices are installed in locations non-condusive to an being attended,
+the remaining option is to not use Full Disk Encryption (FDE).
+
+It may be technically possible to create multiple images (FDE and non-FDE);
+however, this will complicate support options (e.g., doubling the number of
+disk images that need to be built, tested, and maintained).
+
+## Decision
+
+Deployed devices will not use Full Disk Encryption (FDE).
+
+## Consequences
+
+Everything stored on the device will either be unencrypted (i.e., plain-
+text) or incorporate a means to unencrypt data.  This includes source code
+(likely a non-issue given the source code is publicly available) and
+operating system code (similarly available to the public); however, it
+will also include sensory data (even if stored ephemerally) as well as
+credentials (e.g., API keys) used to interact with the system backend.