nitrogen6*_defconfig: bump u-boot and kernel revisions

This update brings several improvements such as:
- switch to a generic Boundary defconfig
  - same across all the Boundary Devices platforms
- support of HDMI to MIPI daugther board
https://boundarydevices.com/product/bd_hdmi_mipi/
- enhanced display support
  - LCD ASIT500MA6F5D
  - LCD A030JN01
  - HDMI 480p

For a complete list of changes:
https://github.com/boundarydevices/linux-imx6/commits/d7a412b
https://github.com/boundarydevices/u-boot-imx6/commits/a4070a8



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