Add a syslinux option to start accessibility

Originally created by @meganet on #7510 (Redmine)

To solve the problem of accessibility tools on greeter and on session start, a solution (using in Debian Wheezy) could be to add an boot option to auto start these tools.

Related issues

Related to #7500 (closed)
Blocked by #6015 (closed)

Edited May 15, 2020 by meganet

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