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

board/boundarydevices/nitrogen6x/readme.txt

@@ -7,3 +7,7 @@ To install, simply extract rootfs.ext2 to first partition of a uSD card:
 sudo dd if=output/images/rootfs.ext2 of=/dev/sdX1
 
 Where 'sdX1' is the device node of the uSD partition.
+
+To upgrade u-boot, cancel autoboot and type:
+
+run upgradeu

configs/nitrogen6x_defconfig

@@ -16,7 +16,7 @@ BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BOARDNAME="nitrogen6q"
 BR2_TARGET_UBOOT_FORMAT_IMX=y
 BR2_TARGET_UBOOT_CUSTOM_TARBALL=y
-BR2_TARGET_UBOOT_CUSTOM_TARBALL_LOCATION="http://github.com/boundarydevices/u-boot-imx6/tarball/63ae4a5/u-boot-63ae4a5.tar.gz"
+BR2_TARGET_UBOOT_CUSTOM_TARBALL_LOCATION="http://github.com/boundarydevices/u-boot-imx6/tarball/f870252/u-boot-f870252.tar.gz"
 
 # kernel
 BR2_LINUX_KERNEL=y