external toolchain: fix sysroot location if the toolchain was moved

Sysrooted toolchain can be relocated. In this case, the sysroot is no
longer located at the place it was configured at.

Signed-off-by: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Acked-By: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>