toolchain/crosstool-NG: fix sysroot name

We have to ensure that the sysroot created while building the
toolchain will be the one we later search for libraries. So:
 - hide the sysroot name prompt from the ct-ng menu.
 - force the sysroot name to be 'sysroot'.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>