toolchain: copy_toolchain_lib_root: copy symlinks instead of recreating them

copy_toolchain_lib_root handles symlinks by recreating them, disregarding
the original destination and assuming the destination is in the same
directory as the link itself.
When a library link points to the real library file in another directory,
for example:
    usr/lib/octeon2/libcrypt.so -> ../../../lib32/octeon2/libcrypt.so.1
then the link created by copy_toolchain_lib_root is broken.

It is more robust to copy the symlink to keep the destination intact. The
destination path should be present, possibly through other symbolic links.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>