tests/boot-serial-test: Add support for the mcf5208evb board

We can output a character quite easily here with some few lines of
assembly that we provide as a mini-kernel for this board.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1512031988-32490-4-git-send-email-thuth@redhat.com>
[lv: add boot-serial-test in check-qtest-m68k]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>