salix-1 is the name of decrypted /dev/sda and after writing this post I changed this parameter to full directory -r /dev/mapper/salix-1. And I did not generate it because I want to be sure that parameters are written correct.

Because of lack of documentation I gave up on setting system with ...

In the /lib/modules/ I have proper 4.4.19 directory.

I was launching this script without any options I didn't know that it have options. Now I run it with -k 4.4.19 and it generates me a command:
mkinitrd -c -k 4.4.19 -f btrfs -r salix-1 -m usb-storage:ehci-hcd:ehci-pci:xhci-pci:ohci-pci:xhci-hcd ...

@ westms
Yeah I know that's why I wanted to install grub but thanks to your guide I will give LILO a try. Because with LILO I can't encrypt /boot I decided to modify my scheme. Now i have luks encrypted btrfs but with luksheader and /boot partition on another device.

I followed this guide but when ...

Thanks for your reply,
I forgot to say that i don't have UEFI so in the partitiom scheme I have bios-boot partition instead. I've been thinking that maybe UEFI is neccesery for this partition scheme to work but I found no info about that.

Hello,
I've followed this guide to do fde with btrfs:
https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_entire_system#Btrfs_subvolumes_with_swap
And my problem is "how can i install Salix into this?". I tried to install Salix with installer to other partition and then cp -a it to my ...