cmd: source: Use script from default config

When sourcing FIT images, source the script node from the "bootscr"
property within the default configuration. If
board_fit_config_name_match is overridden, this will determine the
selected configuration rather than the default one.

The old behaviour is inconsistent with the FIT image specification which
does not mention a "default" property in the "/images" node.

Signed-off-by: Sven Schwermer <sven@svenschwermer.de>