f_sdp: Retry sourcing default FIT script

"script@1" is not a valid node name if the "reg" property is missing.
The yocto build system embeds the boot script as an image that's pointed
to by the "bootscr" property within the default configuration. That is
what we use here as the fallback when sourcing the "script@1" fails.
This implementation should be backward-compatible with existing ITS
files.

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