Add linker scripts (and any additional required code) to support mcuboot

[jessebraham]

Build environment for the bootloader can be found here:
https://github.com/jessebraham/espressif-mcuboot-docker

[MabezDev]

Whilst I don't think this is strictly a beta blocker perse, I think the earlier we get this is in the better, as there will likely be a number of things we'll have to iron out.

Adding to the beta release milestone.