lisa.utils: Add LayeredMapping
lisa.utils: Add LayeredMapping

This is a mapping that allows accessing a base read-only layer, with a
read-write layer on top. All key writes are directed against the top
read-write layer.

ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa