Fix/reload dct functions

[XuGuohui]

Problem

Particle Update fails to update a Photon/P1 from Device OS <1.2.1-rc.3 to 1.2.1 via DFU mode. This is caused by that after updating the system parts, the address of DCT functions that resided in system parts aren't consistent, so when bootloader tries accessing the DCT after that, it then crashes.

Solution

Dirty a flag after every DFU write operation to the internal flash, so that after updating the system parts, next time when bootloader calls DCT functions, it will reload the DCT functions to make sure that the address of the DCT functions in new system parts are valid.

Steps to Test

1. Build and flash the new bootloader
2. Flash older DeviceOS e.g. 1.0.0 onto the device
3. Reset and go into DFU
4. Write into DCT
5. Write system-part1 and system-part2 of DeviceOS release v1.2.1 without resetting
6. Write into DCT again, this should succeed and not crash the device

Example App

N/A

References

particle-iot/particle-cli#499
https://community.particle.io/t/device-os-1-2-1-and-particle-cli-1-43-fail-in-particle-update/51140

• [bugfix] [Photon/P1] Bootloader correctly re-imports the DCT functions from system firmware after its modification through DFU.