Some steps too take a closer look to AVM's firmware
$ mkdir ~/Firmware; cd ~/Firmware
$ wget https://download.avm.de/fritzbox/fritzbox-7590/other/fritz.os/FRITZ.Box_7590-07.13.image
$ sudo apt install binwalk
$ ls
FRITZ.Box_7590-07.13.image
$ binwalk --signature --term FRITZ.Box_7590-07.13.image
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
0 0x0 POSIX tar archive (GNU)
$ tar xvf FRITZ.Box_7590-07.13.image
./var/
./var/install
./var/regelex
./var/tmp/
./var/tmp/kernel.image
./var/tmp/filesystem.image
./var/info.txt
./var/content
./var/version
./var/chksum
./var/urladerupdate
./var/signature
$ cd tmp/
$ binwalk --signature --term kernel.image
DECIMAL HEXADECIMAL DESCRIPTION
------------------------------------------------------------------------------------------------------------------------------------------------------
4753479 0x488847 LZMA compressed data, properties: 0x5B, dictionary size: 0 bytes, uncompressed size: 2152784640 bytes
ron@laptop-dell:~/Desktop/Firmware/var/tmp$ binwalk --signature --term filesystem.image
DECIMAL HEXADECIMAL DESCRIPTION
------------------------------------------------------------------------------------------------------------------------------------------------------
0 0x0 Squashfs filesystem, big endian, version 4.0, compression:xz, size: 24002361 bytes, 3836 inodes, blocksize: 65536
bytes, created: 1970-10-05 19:19:21
$ binwalk --extract --quiet filesystem.image
WARNING: Extractor.execute failed to run external extractor 'sasquatch -p 1 -le -d 'squashfs-root' '%e'': [Errno 2] No such file or directory: 'sasquatch': 'sasquatch', 'sasquatch -p 1 -le -d 'squashfs-root' '%e'' might not be installed correctly
WARNING: Extractor.execute failed to run external extractor 'sasquatch -p 1 -be -d 'squashfs-root' '%e'': [Errno 2] No such file or directory: 'sasquatch': 'sasquatch', 'sasquatch -p 1 -be -d 'squashfs-root' '%e'' might not be installed correctly
$ git clone https://github.com/devttys0/sasquatch
$ cd sasquatch
$ ./build.sh
..
$ mkdir -p /usr/local/bin
$ cp sasquatch /usr/local/bin
$ binwalk --extract --quiet filesystem.image
$ ls
filesystem.image _filesystem.image-0.extracted _filesystem.image.extracted kernel.image