tests/boot_linux_console: add a test for mips + malta

tests/boot_linux_console: add a test for mips + malta

Similar to the x86_64 + pc test, it boots a Linux kernel on a Malta
board and verify the serial is working.  Also, it relies on the serial
device set by the machine itself.

If mips is a target being built, "make check-acceptance" will
automatically include this test by the use of the "arch:mips" tags.

Alternatively, this test can be run using:

    $ avocado run -t arch:mips tests/acceptance
    $ avocado run -t machine:malta tests/acceptance
    $ avocado run -t endian:big tests/acceptance

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190312171824.5134-14-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>