install.sh: don't clobber dangling symlinks

install.sh: don't clobber dangling symlinks

Symlinks in a subdirectory that is to become target rootfs are sometimes
dangling because they link to canonical file names that are not present
on the host, but are present relative to the target rootfs root. Don't
copy over dangling symlinks when noclobber is enabled

The -e test treats dangling symlinks as non-existent files. Add -h test
that returns true for all symlinks.

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>