tools/config: Allow building of components to be controlled from .config

tools/config: Allow building of components to be controlled from .config

For build systems which build certain Xen components separately, allow certain
components to be conditionally built based on .config, rather than always
building them.

This patch allows qemu and blktap to be configured in this manner.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>