${CT_PREFIX_DIR} may contain relative paths (e.g.
"${CT_TOP_DIR}/../x-tools/${CT_TARGET}"). Code added in commit d83a0036
("Add symlinks from the libraries' original location to the new one.")
didn't cope well with this. As we're already calculating
${cannon_prefix} make use of it when adding the symlinks. This avoids
any issues with ${CT_PREFIX_DIR}.
Fixes#1807
Signed-off-by: Chris Packham <judge.packham@gmail.com>