armvirt: rename config-default to config-4.9

The kernel configs for all targets should have the version in the
filename, for clearness and consistency across all targets.
It is also expected by the update_kernel.sh script.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

diff --git a/target/linux/armvirt/config-default b/target/linux/armvirt/config-4.9
similarity index 100%
rename from target/linux/armvirt/config-default
rename to target/linux/armvirt/config-4.9