u-root.go: test for relative paths with os.Stat()

The current u-root has gotten a little frustrating
with messages like this:
22:35:48 "cpu/cmds/cpud/" is not a valid path, allowed are only existing relative or absolute file paths!

Which is not only wrong, in this case: the path cpu/cmds/cpud does exist and is relative,
but very confusing when you use ./cpud/cmds/cpud instead.

Do the easy thing with the arg to validateArg first: just os.Stat().

If it fails, the more complex test can be used, but if it succeeds, time is saved.

Signed-off-by: Ronald G. Minnich <[email protected]>

u-root.go

@@ -369,6 +369,12 @@ func Main(l ulog.Logger, buildOpts *gbbgolang.BuildOpts) error {
 }
 
 func validateArg(arg string) bool {
+	// Do the simple thing first: stat the path.
+	// This saves incorrect diagnostics when the
+	// path is a perfectly valid relative path.
+	if _, err := os.Stat(arg); err == nil {
+		return true
+	}
 	if !checkPrefix(arg) {
 		paths, err := filepath.Glob(arg)
 		if err != nil {