The exact set of environment variables required by xt-xcc and xt-clang
is a bit of a dark magic and error-prone. TOOLCHAIN_VER is definitely
one of them and the error message was puzzling when TOOLCHAIN_VER was
undefined or not exported.
- Add TOOLCHAIN_VER to the list of variables in the error message
- Replace the vague "toolchain not found" in the error message with the
more useful "XCC/install/tools//XtensaTools/bin/xt-xcc not found" where
the double slash and comparison with the filesystem clearly point at
where TOOLCHAIN_VER is missing.
Signed-off-by: Marc Herbert <marc.herbert@intel.com>
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