ARM: exynos_defconfig: Set recommended options for systemd

Set following options to a value recommended by systemd (which also
matches the multi_v7 deconfig):
1. Enable AUTOFS4_FS - for systemd.automount [0];
2. Enable BLK_DEV_BSG - SG v4 for recend udev [0][1];
3. Disable UEVENT_HELPER_PATH - legacy hook for hotplug, forked for each
   uevent, slows down booting [0];

[0] http://cgit.freedesktop.org/systemd/systemd/tree/README
[1] http://patchwork.ozlabs.org/patch/47921/



Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>