toolchain-scripts: Fix TARGET_SYS reference

There was a fix missing from a previous commit:

http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=20a86e16bc54b74fbdb2a3a407d54210ea262925

since there was another section of toolchain-scripts needing updating.
This patch fixes the missing reference and unbreaks the ADT toolchain.

[YOCTO #5340]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
build-appliance: Update SRCREV for release

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>