Sub flavours are flavours that are built based on other builds. IOW, they
are usually a subset of something else.

Requirements:

debian/sub-flavours/<sub>.list	: The file list, uses glob syntax
debian/sub-flavours/<sub>.vars	: The make vars, similar to normal flavours
debian/rules.d/<arch>.mk	: Add <base>_sub var listing the <sub>, e.g.
				  server_sub = virtual would mean virtual is
				  based on the server flavour.

Note, the vars must include a conflicts with the flavour it was built on.