[MINIOS] Clean up final link stage of the build.

[MINIOS] Clean up final link stage of the build.

Allow extra application objects to be linked against a partial link of
minios with non-global names localised. For now we assume that global
names ill be assigned a xenos_ prefix in due course. We may decide to
choose a different prefix.

Those who wish to finish the link from an external Makefile can link
their own objects against mini-os.o (which must be first in link
order!) and specify the appropriate architecture linker script.

Signed-off-by: Keir Fraser <keir@xensource.com>