nitrogen6x_defconfig: bump u-boot to version 63ae4a5

Bump u-boot to latest version binary provided by Boundary Devices ([1]).

This version is only 6 commits behind the head of the git production
branch ([2]) recommended by Boundary Devices ([3]).

Tested on nitrogen6q-rev3 board.

[1] http://commondatastorage.googleapis.com/boundarydevices.com/u-boot-production.tar.gz
[2] https://github.com/boundarydevices/u-boot-imx6/commits/production
[3] http://boundarydevices.com/i-mx6-frequently-asked-questions-and-answers/#latest-uboot



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>