[new uImage] Add new uImage format support to autoscript routine

autoscript() routine is updated to accept second argument, which
is only used for FIT images and provides a FIT subimage unit name.

autoscript() routine callers must now pass two arguments. For
non-interactive use (like in cmd_load.c, cmd_net.c), new environment
variable 'autoscript_uname' is introduced and used as a FIT
subimage unit name source.

autoscript command accepts extended syntax of the addr argument:
addr:<subimg_uname>

Signed-off-by: Marian Balakowicz <m8@semihalf.com>