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>

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x"

     fi

 }