mkimage: fix Segmentation Fault when run without "-n name" option

The restructuring of the mkimage command in commit 89a4d6b1 ("tools:
mkimage: split code into core, default and FIT image specific")
introduced a bug that caused mkimage to segfault when run without
"-n name" option.  Initialize the imagename entry to prevent that.

Signed-off-by: Wolfgang Denk <wd@denx.de>