Tracking: detached swap image creation

[bunnie]

Tracking for the first item on https://github.com/betrusted-io/xous-core/pull/524

• [x] Add key to Swap karg tag to track encryption key for encrypted swap
• [x] Modify MiniElf format to comprehend encrypted swap (needs to have a separate base from FLASH memory for tracking ELF files located in encrypted swap)
• [x] Define nonce format for AEAD. I think deterministic nonces are fine for this?
• [x] Define AAD, if any, for AEAD. Probably good to have some domain separation, no?
• [x] Modify image writing to encrypt images using an AEAD
• [x] Header for tracking detached MAC data on the swap blob
• [x] Modify the Precursor target to handle swap blob (in memory-mapped FLASH memory)
• [x] Modify Cramium target to handle swap blob (in non-memory mapped UDMA-based SPI memory)

[bunnie]

AEAD/AAD notes updated here: https://betrusted.io/xous-book/ch10-00-swap-overview.html#image-creation

[bunnie]

This is now ready for review in #524