This may sound like a silly question, how do I accept a certificate while SVN is running?
Shouldn't it do this automatically for me?
Not silly at all. Outside the OpenWrt compilation you can issue the same command, in this case
svn co -r1383 https://dev.wifidog.org/svn/trunk/wifidog
and accept the certificate permanently. And browsing the SVN web page should work too.
Better info in
http://svnbook.red-bean.com/en/1.5/svn.serverconfig.httpd.html#svn.serverconfig.httpd.authn.sslcerts Saludos, Julio