Run sb-check early in the build scripts.

[c-mauderer]

The build scripts should call sb-check as soon as possible to warn the user if there are any missing prerequisites.

[c-mauderer]

It is also necessary to check for waf. See also #4

[kiwichris]

Note: https://git.rtems.org/rtems-source-builder/tree/source-builder/sb/setbuilder.py#n538 does the same thing as sb-check each time sb-setbuilder runs.