nitrogen6x_defconfig: bump u-boot version

To the 2014.07 release. For furture details, see:

http://boundarydevices.com/u-boot-2014-07-release-mx6-boards/



Also update our 6x_upgrade script to the version from that page so the
entire SPI flash gets erased (otherwise it fails on big u-boot builds) and
document how to perform the upgrade.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

u-boot fixup