Switch armhf to use common default Linux.

Switch armhf to use common default Linux.

Previous we had a v3.13 based branch which was used for ARM (because
v3.4 was too old). Now that the default is v3.14 we can use it.

standalone-generate-dump-flight-runvars before and after shows the
build-armhf-pvops jobs' linux_revision runvar changing from
95bfbee422b9b1cfe8c2d2e27edf17ce1cc99e04 (v3.13-63-g95bfbee) to
946de0e6b6ed49eacb03e3cddfcb1d774d6378ed (v3.14.17) for these branches:

	libvirt
	osstest
	qemu-mainline
	qemu-upstream-unstable
	xen-4.3-testing
	xen-4.4-testing
	xen-unstable

This is as expected.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>