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>

 reporter_name="YEM"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment=""