Boot Linux EFI without intermediate bootloader

update-kernel

#!/usr/bin/bash
# How to add a UEFI entry
# efibootmgr \
#  --disk /dev/sda \
#  --create \
#  --label 'Choisis moi' \
#  --loader '\EFI\efistub\vmlinuz' \
#  --unicode 'root=UUID=50200856-c21a-4bea-9412-bd2f7b27b8b5 ro rootflags=subvol=root rhgb quiet initrd=\EFI\efistub\initramfs' \
#  --verbose

VMLINUZ=$(ls /boot/|grep "vmlinuz"|grep -v "rescue"|tail -n1)
INITRAMFS=$(ls /boot/|grep "initram"|grep -v "rescue"|tail -n1)
EFIPART="/dev/sda1"
EFIMOUNT="/mnt/ssd-efi"

echo "Selected kernel: $VMLINUZ"
echo "Selected ramfs: $INITRAMFS"

mkdir -p $EFIMOUNT
mount $EFIPART $EFIMOUNT

echo "Start copy"
cp /boot/$VMLINUZ $EFIMOUNT/EFI/efistub/vmlinuz
cp /boot/$INITRAMFS $EFIMOUNT/EFI/efistub/initramfs

echo "Copy terminated"
umount -l $EFIMOUNT

echo "Done. Have fun."

update-kernel.service

[Unit]
Description=Update Kernel

[Service]
Type=oneshot
RemainAfterExit=true
ExecStop=/usr/local/sbin/update-kernel

[Install]
WantedBy=multi-user.target