mkimage: fit: handle FDT_ERR_NOSPACE when ciphering

Also replace fdt_delprop/fdt_setprop with fdt_setprop as fdt_setprop can
replace an existing property value.

Signed-off-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>