Fix the handling of legacy-format devices in the u-boot loaderdev variable.
When I added support for the standard loader(8) disk0s2a: type formats,
the parsing of legacy format was broken because it also contains a colon,
but it comes before the slice and partition. That would cause disk_parsedev()
to return success with the slice and partition set to wildcard values.

This change examines the string first, and if it contains spaces, dots, or
a colon at any position other than the end, it must be a legacy-format
string and we don't even try to use disk_parsedev() on it.

Reported by: Manuel Stuhn