bootloader: remove "Dir()" from Bootloader interface
The Bootloader.Dir() method is no longer needed and can be removed.
It used to be used to extract kernel assets but that is now part
of the Bootloader interface so expect for test code nothing is
using "Dir()" anymore. The extraction of boot assets is now done
via the Bootloader interface directly.
Note that Dir() also does not make sense for bootloaders that work
inside partitions (like littleKernel).
overlord: mostly stop needing Kernel/CoreInfo, make GadgetInfo consider a DeviceContext
In a world with remodeling there might be more than one kernel, gadget etc on the device, one for the current model and one for model to-be. So the <deviceSnap>Info helpers are misleading.
These changes clean up things in the area, remove Kernelnfo, and make GadgetInfo and derived helpers take a DeviceContext.
In the same context these make CoreInfo internal only to snapstate. Because "core" vs "snapd" as system snap, new code shouldn't take "core" as preeminent.
