Hello
First of all, I am french and prefered to write in english because this method is not difficult to understand 
I found a basic way to deactivate menu selection on startup (selection between normal or verbose)
Recalbox - boot without any linux progress and launch video splash
Recalbox (verbose) - show linux progress and launch video splash
On my case, I use Recalbox on PC but this can be done from SD or USB stick
From HDD or SSD
If you already use Recalbox and avoid mount HDD - SSD on your PC
connect to your machine via SSH
mount -o remount,rw /boot
cd /boot
ls
EFI hardware.log
System Volume Information recalbox-backup.conf
asound.state recalbox-boot.conf
boot update
boot.lst
if you have configured your machine on UEFI
cd EFI
ls
BOOT
cd BOOT/
ls
bootia32.efi bootx64.efi grub.cfg
if you don't have your machine configured on UEFI
cd boot
ls
grub initrd.gz linux recalbox
cd grub
ls
grub.cfg
grub.cfg is the file about menu selection and time for selection
to edit this file
nano grub.cfg
you will see this
set default="0"
set timeout="5"
set menu_color_normal=cyan/blue
set menu_color_highlight=white/blue
menuentry "Recalbox" {
set gfxpayload=auto
linux /boot/linux label=RECALBOX console=tty3 quiet loglevel=0
initrd /boot/initrd.gz
}
menuentry "Recalbox (verbose)" {
set gfxpayload=auto
linux /boot/linux label=RECALBOX console=tty1 loglevel=7
initrd /boot/initrd.gz
}
replace value set timeout="5" by "0"
make CTRL+X to leave
push Y to confirm save
press ENTER to confirm the name and exit nano
reboot your system to see the change
have a good game
