setenv(): Delete 0-length environment variables

Previously setenv() would only delete an environment variable if it
was passed a NULL string pointer as a value.  It should also delete an
environment variable when it encounters a valid string pointer of
0-length.

This change/fix is generally useful and is necessary for the upcoming
"editenv" command.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

common/cmd_nvedit.c

@@ -400,7 +400,7 @@ int _do_setenv (int flag, int argc, char *argv[])
 int setenv (char *varname, char *varvalue)
 {
 	char *argv[4] = { "setenv", varname, varvalue, NULL };
-	if (varvalue == NULL)
+	if ((varvalue == NULL) || (varvalue[0] == '\0'))
 		return _do_setenv (0, 2, argv);
 	else
 		return _do_setenv (0, 3, argv);