qemu/x86: update defconfig to Linux 3.15.x

Update the default configuration to use Linux 3.15.x.
Enable ext4, so that ext2/ext3/ext4 can be used.
Tested with Qemu 2.0.0.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>