ap-common: Switch to Linux 4.14 by default on X86.

ap-common: Switch to Linux 4.14 by default on X86.

Linux 4.9 is getting a bit long in the tooth.  4.14 is an LTS branch
and the osstest-tested version seems reasonably good.  I ran a special
report[1] to see what to expect and it reported no regressions.

Accordingly I am going to switch to using Linux 4.14 by default for
most X86 runs in osstest.  ARM tests are not affected at this time;
they use their own linux-arm-xen branch which is updated by the Xen
ARM maintainers.

[1] ./sg-report-flight --that-linux=6ba89b52ba6916bc7a3d390d70951e992c0ca39e --this-linux=d88700f79448fc8f03617d4f1929c39676f8d1e4 --branches-also=linux-4.9,linux-4.14,linux-arm-xen 122974 |less

CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: Juergen Gross <jgross@suse.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>