Fix build with variants enabled

When build variants are enabled (even if they're not used for this
driver) executables may be built inside a "novariant" toolchain. That
wasn't accounted for correctly in the paths, so in particular we need to
switch to using root_build_dir instead of root_out_dir for finding
files. We also need to explicitly mention the toolchain.

Change-Id: I6b019bad887d77dad994bb12eb473775de423365
1 file changed