arm/run: Allow Migration tests

Let's link getchar.o to use puts and getchar from the
tests.

Then allow tests belonging to the migration group to
trigger the migration from the test code by putting
"migrate" into the uart. Then the code can wait for the
migration completion by using getchar().

The __getchar implement is minimalist as it just reads the
data register. It is just meant to read the single character
emitted at the end of the migration by the runner script.

It is not meant to read more data (FIFOs are not enabled).

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>