default.xml: select 2012.07 u-boot revision tag

Use a newer u-boot release to support usb commands, which
provide ethernet connectivity. This allows for tftpboot
to be used.

Change-Id: I4ba6b621546ae228e61e7878242b7e8e4102fbd7
Signed-off-by: Arend van Spriel <arend@broadcom.com>