swig: couple of buildsystem fixes

[MilhouseVH]

I noticed that libcec and kodi on Ubuntu were being built with the build host swig (3.0.8 on 16.04, 3.0.10 on 17.10) from /usr/bin, and not $TOOLCHAIN/bin/swig which is currently version 3.0.12.

The reason for this is that Ubuntu has:

neil@nm-linux:~/projects$ ls -la /usr/bin/swig*
lrwxrwxrwx 1 root root       7 Jul 16 20:06 /usr/bin/swig -> swig3.0
-rwxr-xr-x 1 root root 1765416 Jul 16 20:06 /usr/bin/swig3.0

while we only have $TOOLCHAIN/bin/swig, and find_package(SWIG) in libcec (and also kodi) is looking for and finding swig3.0 before falling back to swig, so consequently it ignores our toolchain binary and uses the build-host file.

This PR fixes this error by configuring swig to create the swig3.0 binary in our toolchain.

The swig sym link is just for backward compatibility (ie. just in case), same as Ubuntu.

Thanks @Raybuntu for looking into this.

[Ray-future]

Thanks