Add image_arch and qemu_required options

[llamasoft]

The inclusion of image_arch improves detecting when qemu is required (#145). The image architecture is also used to intelligently determine which flavor of qemu is required, if any. Additionally, the qemu_required option had been added which forces the use of qemu even when it's not stricly required by qemu_args, qemu_binary, or image_arch. This aims to help in build situations where the user wants reproducible emulation or when QEMU_* environment variables need to be respected. The default values for both of these options have been chosen to prevent any breaking changes to existing configurations. One example configuration has been updated to enable native image building should the build environment support it.

[solo-build-bot[bot]]

Waiting for approval from someone in the solo-io org to start testing.