toolchain-external: hook for Codescape toolchain side-by-side layout

toolchain-external: hook for Codescape toolchain side-by-side layout

The Codescape toolchain uses a sysroot layout that places them
side-by-side instead of nested like multilibs. A symlink is needed much
like for the nested sysroots which are handled in copy_toolchain_sysroot
but there is not enough information in there to determine whether the
sysroot layout was nested or side-by-side.

For the above reason plus the fact that this is the only toolchain
needing this, better to handle that symlink creation using a hook which
will be executed only when that toolchain is selected.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>