sd-boot: rename ConfigEntry field 'file' to 'id'

sd-boot: rename ConfigEntry field 'file' to 'id'

The field derives from a file name only in very specific cases, for
many cases it's a fixed string (for example, all "auto-" items are like
this). Also, even when it derives from a file name, it is processed a
bit, as suffixes are removed and the string is converted to lower case.

hence, let's name this field "id" instead, because that's what it is
used for: as general identification token.