Rockbox

There were two problems with svnversion.sh@14659:
- If it was run on a machine with localization set to anything other than
English, the comparison to "exported" would get confused and print something
like "rexportiert -070910". This caused build problems because the string
isn't quoted before being passed to gcc.
- If svnversion fails (such as when invoked with a non-existent directory),
the output would look like "r-070910" because svnversion.sh didn't check for
command failure.

This patch fixes both issues, and refactors svnversion error and output handling
into a shell functions.

Thanks to Rincewind, GodEater_ and amiconn for discovering the bug, helping fix
it and test the fix.