config: add an option to name the sysroot directory

Depending on local policies, some users have expressed a need to
have the sysroot be named differently than the hard-coded name.

Add an option for that.
Default to 'sysroot' to match the existing literature.

While at it, replace 'sys-root' with 'sysroot' everywhere we
reference the sysroot.

Reported-by: Alexey Kuznetsov <Alexey.KUZNETSOV@youtransactor.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

#!/bin/sh

call_to=@@DEFAULT_LD@@

case "${CTNG_LD_IS}" in

    bfd)    call_to=bfd;;

    gold)   call_to=gold;;

esac

exec "${0}.${call_to}" "$@"

exit $?