wandboard_defconfig: bump u-boot to 2015.07 and kernel to 3.14.28_1.0.0

Bump U-boot version to 2015.07 from the mainline repository. Since there
is SPL support for the wandboard in the U-boot 2015.07, it is now
possible to boot the same U-boot image for all wandboard variants
(solo/dual/quad).

Bump kernel version to 3.14.28_1.0.0 from the wandboard git repository.

Tested on Wandboard Solo, Wandboard Dual and Wandboard Quad.

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>