alarmdecoder.com

These instructions seem irrelevant with latest version of Image build 2018-05-29
The called out entries and inittab is not there.
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Remove the following entries from /boot/cmdline.txt:

console=ttyAMA0,115200
kgdboc=ttyAMA0,115200

Remove the following line from /etc/inittab:

T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

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If you are using the alarmdecoder image, that is already done.

Right, the instructions look like it's for using the image.. So, redundant.

No, those are the manual install instructions for people who aren't using the image.