In the bootloader, deinitialise all activated peripherals before jumping to main app

[amrbekhit]

Following on from this mailing list thread, any peripherals turned on in the bootloader remain on after the main app has started executed, causing unexpectedly high power consumption. I think it would be a good idea for the bootloader to de-initialise any initialised peripherals before jumping the main app. This would allow the main app to start from a blank slate and removes the need to add any de-initialisation code to it (which would require knowing what the bootloader has initialised).

[sjanc]

Since we deleted bootloader from core and moved to mcuboot I think this can be closed (and tracked in mcuboot instead if relevant)