buildroot

configs/nitrogen*: bump kernel and u-boot revisions

Here are the main U-Boot modifications:
- Rebase on top of mainline 2017.03 release
- Fix sparse image flashing issues

Here are the main Linux modifications:
- Add mma8451 driver support
- Fix possible TiWi spurious interrupts issue
- Fix Ethernet interrupt parent issue
- Add USB Type C TUSB320 controller support
- Add SOM2 i.MX6DL and i.MX6QP variants

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