implement accessibility support in archiso

this fixes #67 (closed)

what's done

1. added needed packages:
   • alsa-utils: sound support
   • brltty: braille terminal support
   • espeakup: console screen reader
2. added bootloader configuration (note 1)
3. implemented helper script (note 2)
4. implemented helper script startup logic
5. increased bootloader timeout (more info in 8bcea215)
6. adjusted zsh option to improve screen reader support (more info in 01909c93)

implementation ditales

note 1

a new bootloader entry is added. this entry contains a kernel cmd line option witch red by systemd to enable A11Y unit.

note: this option has no effect on kernel and only serves as an indication flag.

note 2

livecd-sound:

helper script which can:

• set all ALSA cards to max volume at boot.
  • started by livecd-alsa-unmuter.service unit if kernel cmd line flag is set.
• select a sound card with Auditory feedback.
  • started by livecd-talk.service unit

livecd-talk.service also starts screen reader.