stdlib: Create the AbstractSystemBoard

In the prior design the AbstractBoard inherited from System. This was
fine for the X86Board and RISCVBoard, but was causing problems for the
design of the ARMBoard which must inherit from ArmSystem (which itself
inherits from System).

This new design removes the System inheritance from AbstractBoard, and
creates a new "AbstractSystemBoard" class which inherits from both
AbstractBoard and System. This allows for the creation of an ArmBoard
which inherits from both AbstractBoard and Arm System.

Change-Id: I3831f2ebe61e4adfb9dc10b7790b39403a99beaa
Tested-by: kokoro <>
Reviewed-by: Bobby Bruce <>
Reviewed-by: Jason Lowe-Power <>
Maintainer: Bobby Bruce <>
7 files changed