[rom_ext] Migrate to ECDSA verification of application firmware

Migrate to ECDSA verification of application firmware.

Currently, the ROM_EXT uses RSA3K keys to validate application firmware, but we want to use ECDSA verification instead and eliminate the use of RSA keys.

- [ ] Migrate the ES ROM_EXT to using ECDSA keys
- [x] Cherry-pick ECDSA changes from `master` branch.
- [x] Integrate ECDSA sigverify into the ROM_EXT.
- [x] Change FPGA application keys to ECDSA keys.
- [x] Change SiVAL application keys to ECDSA keys.
- [x] Change ProdA application keys to ECDSA keys.
- [x] Change ProdC application keys to ECDSA keys.
- [x] Eliminate RSA application keys from the `earlgrey_es_sival` branch.
- [ ] Cherry-pick the ROM_EXT changes and new keys to `master`
- [ ] Cherry-pick the ROM_EXT changes and new keys to `earlgrey_1.0.0`