amazon-austin: Add README

devices/amazon-austin/README.adoc

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+= Amazon Fire 7 (7th generation) (2017)
+include::_support/common.inc[]
+
+== Device-specific notes
+
+[NOTE]
+====
+*Do not acquire any Amazon Fire devices with the intent to run Mobile NixOS on
+it.*
+
+They are *not* recommended devices. They are not designed to allow the end-user
+to run customized operating systems.
+====
+
+This works on top of the *amonet* hack, which allows unsigned images to be
+booted on some Amazon Fire devices.
+
+* https://forum.xda-developers.com/t/unlock-root-twrp-unbrick-downgrade-fire-7-ford-and-austin.3899860/
+
+Flashing to `amazon-austin` requires partially disassembling the device at
+least once to exploit a boot ROM flaw.
+
+
+=== Flashing
+
+WARNING: **DO NOT** flash on top of the hacked TWRP. As it is the image is not
+modified to hold the "hacked" payload, which means recovery would be unusable.
+
+Flashing the boot image requires unergonomic steps.
+
+First, push the built image on the device (e.g. in the temporary rootfs).
+
+```
+ $ adb push /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mobile-nixos_amazon-austin_boot.img /boot.img
+/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mobile-nixos_amazon-austin_boot.img: 1 file pushed, 0 skipped. 7.4 MB/s (13455360 bytes in 1.729s)
+```
+
+Then, `dd` it in the appropriate partition. Make sure you copy the exact given
+command. Flashing to the wrong partition could (temporarily) brick the device.
+
+```
+ $ adb shell dd if=/boot.img bs=1M of=/dev/block/platform/mtk-msdc.0/by-name/boot
+12+1 records in
+12+1 records out
+13455360 bytes (12.8MB) copied, 0.543268 seconds, 23.6MB/s
+```
+
+At this point, the image is flashed unmodified. It will not be bootable. To
+make the image bootable, boot back once into the hacked TWRP. At boot, it will
+apply the necessary modifications to the boot image.
+
+```
+ $ adb shell reboot recovery 
+```
+
+At this point, you can reboot normally.