fastboot: only look up real partition names when no alias exists

Having U-Boot look up the passed partition name even though an alias
exists is unexpected, leading to warning messages (when the alias name
doesn't exist as a real partition name) or the use of the wrong
partition.

Change part_get_info_by_name_or_alias() to consider real partitions
names only if no alias of the same name exists, allowing to use aliases
to override the configuration for existing partition names.

Also change one use of strcpy() to strlcpy().

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>