Buster and 1.9.4 upgrade + bugfixes

[psporysz]

I tried to use existing binary with following results:

Nov 12 09:02:59 raspberrypi S02greengrass[2408]: mkdir: cannot create directory ‘/greeng
Nov 12 09:02:59 raspberrypi S02greengrass[2408]: cp: failed to access '/greengrass/certs
Nov 12 09:02:59 raspberrypi S02greengrass[2408]: cp: cannot stat '/boot/config/*': No su

Then i've build it from sources but unfortunately the result was the same. Once i've sorted out wrong paths new thing has pop out:

Nov 12 10:06:56 raspberrypi S02greengrass[1963]: Found cgroup subsystem: pids
Nov 12 10:06:56 raspberrypi kernel: overlayfs: overlapping upperdir path
---
failed to create overlay fs for container nosysRootfs too many levels of symbolic links

I found this and this threads so decided to upgrade the pi-gen and Greengrass to working numbers.

Beside the above i've also reverted original pi-gen files to streamline future merges and have improved the documentation slightly.

I've replaced AWS certificate with recommended one which is probably sane default for newcomers who will try to use the service in AWS.

Hope all above makes sense :)