Variable functionality

Most of the variable functionality in the runtime services are now in place. Implemented as a runtime module they utilize the variable store protocol for backing storage. Currently only a memory store is implemented that can keep variables for duration of boot- and runtime services. The next step is to implement a non-volatile store that keeps the variables in a special file on the system partition.

An open question is how to detect which partition is the system partition. If the disk uses the GPT scheme the partition can be detected using the partition GUID. But what about disks with MBR partitions? Should the partition that contains a \efi directory be picked? Should some other algorithm be used? Suggestions are welcome.