toolchain-shar-extract.sh: improve behaviour when xz is not installed
Currently if xz isn't installed when installing the SDK you get an error
from tar and then the SDK installation continues, cascading into a load
of other failures since files are missing. Add a check to see if xz can
be run and error out if it can't, and also exit if tar fails.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
