This allows the URL used to clone our specific version of u-boot to be
specified in the environment, instead of being hard-coded.
This new flexibility will be required by the new Continuous Integration
infrastructure, whose test nodes are in a network without direct
internet access.
Change-Id: I7440b5bba6786f979623b4509111e4e99c6558f6
Replaced the wget download of u-boot by a versioned git checkout
this allows us to better manage the u-boot and MLO version we ship
while still allowing us to build ofline.
This changes replaces the BASE_URL setting by U_BOOT_BIN_DIR and
also updates to a newer build of u-boot.