external-toolchain: only copy existing directories of the sysroot

Instead of copying all directories in "etc lib sbin usr", check that
each of them exists before doing the copy. This is only to avoid an
harmless error message.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>