binman: Allow selecting default FIT configuration

Add a new entry argument to the fit entry which allows selection of the
default configuration to use. This is the 'default' property in the
'configurations' node.

Update the Makefile to pass in the value of DEVICE_TREE or
CONFIG_DEFAULT_DEVICE_TREE to provide this information.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Michal Simek <michal.simek@xilinx.com>