This commit provides a method to verify that firmware has been signed with a private key given its public key. The implementation uses ed25519-dalek as the signature verifier. An "ed25519" feature is required to enable the functionality. When disabled (the default), calling the firmware updater's verify method will return a failure.

* Add nRF91 as target in CI builds
* Add example linker scripts for nrf91
* Make less nRF52 assumptions example config
* Add llvm-tools-preview required for cargo objcopy example

The memory layout of the s140 crept into a number of memory files, which can cause confusion.

This should remove some confusion around embassy-boot-* being a library
vs. a binary. The binary is now an example bootloader instead.