toolchain-shar-extract.sh: Handle special characters in script path

Extracting SDK archive may fail if the script is run using a path with
special characters such as space or asterisk. This is because the shell
interprets such characters after expanding the $0 variable.

Added quotes to all uses of the shell variable $0 to fix this.

Signed-off-by: Andrey Mozzhuhin <amozzhuhin@yandex.ru>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>