sd-boot: add initramfs hook

From kernel-team.pages.debian.net/kernel-handbook/ch-update-hooks.html and bugs.debian.org/826045#51

Unfortunately with this script installed the initramfs hook is run before the kernel one, and I don't know how to avoid this. I believe it's not a big deal though.

# apt install --reinstall linux-image-5.18.0-2-amd64
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
Need to get 69.5 MB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://localhost:3142/deb.debian.org/debian testing/main amd64 linux-image-5.18.0-2-amd64 amd64 5.18.5-1 [69.5 MB]
Fetched 69.5 MB in 1s (121 MB/s)                       
(Reading database ... 265312 files and directories currently installed.)
Preparing to unpack .../linux-image-5.18.0-2-amd64_5.18.5-1_amd64.deb ...
Unpacking linux-image-5.18.0-2-amd64 (5.18.5-1) over (5.18.5-1) ...
/etc/kernel/postrm.d/systemd-boot:
Removing kernel version 5.18.0-2-amd64 from systemd-boot...
Setting up linux-image-5.18.0-2-amd64 (5.18.5-1) ...
/etc/kernel/postinst.d/dkms:
dkms: running auto installation service for kernel 5.18.0-2-amd64:.
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.18.0-2-amd64
Updating kernel version 5.18.0-2-amd64 in systemd-boot...
/etc/kernel/postinst.d/systemd-boot:
Installing kernel version 5.18.0-2-amd64 in systemd-boot...