eclipse support: document script and add checks

As requested by Peter, add a bit of documentation in the
eclipse-register-toolchain script, and add a few more checks (even
though this script is not intended to be executed manually, which is
also now mentionned in the documentation).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>