sysctl: remove index() for buildroot

The uClibc does not support index() function.

A patch from Buildroot.

Reference: http://git.buildroot.net/buildroot/patch/?id=f460603f02092b4f514d108dd8b4167a7a056213
Backported-by: Sami Kerola <kerolasa@iki.fi>

sysctl.c

@@ -297,7 +297,7 @@ static int WriteSetting(const char *setting) {
       return 0;
    } /* end if */
 
-   equals = index(setting, '=');
+   equals = strchr(setting, '=');
  
    if (!equals) {
       fprintf(stderr, ERR_NO_EQUALS, setting);
@@ -546,7 +546,7 @@ int main(int argc, char *argv[]) {
          if (NameOnly && Quiet)   // nonsense
             return Usage(me);
          SwitchesAllowed = false;
-         if (WriteMode || index(*argv, '='))
+         if (WriteMode || strchr(*argv, '='))
             ReturnCode = WriteSetting(*argv);
          else
             ReturnCode = ReadSetting(*argv);