u-boot: 2015.04 -> 2015.07

A new regularly release. Some improvements I've noted:
    - Keyboard on the pcDuino3 Nano now works without a hub.
    - Ctrl-C now correctly cancels the 'sysboot' boot menu

Also, config_cmd_default.h is replaced by equivalents in the kconfig
system, so the vexpress patch needs some updating.