fix the script usage message

The boot.sh script has been updated to accept a test case and an
optional path to a distro image as its parameters. But the
script's help message has not been updated accordingly. Fix the
help message accordingly.

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
Change-Id: Iac4435a1382d55533c0f616fd7cee4990f1cca8e