configs/nitrogen6*: bump kernel version

Update to latest boundary-imx_3.14.28_1.0.0_ga commit which brings the
following additions:
- Fix cache functions export when !MULTI_CACHE
https://github.com/Freescale/linux-fslc/commit/6325d57


  - Mandatory for Vivante kernel module
- Add TW686x video decoder support
- Fix OV5642 subdev/capture driver conflict
- Add GSLX680 touchscreen support

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>