kern/main: Suppress the "Welcome to GRUB!" message in EFI builds

kern/main: Suppress the "Welcome to GRUB!" message in EFI builds

GRUB EFI builds are now often used in combination with flicker-free
boot, but this breaks with upstream GRUB because the "Welcome to GRUB!"
message will kick the EFI fb into text mode and show the msg, breaking
the flicker-free experience.

EFI systems are so fast, that when the menu or the countdown are
enabled the message will be immediately overwritten, so in these cases
not printing the message does not matter.

And in case when the timeout_style is set to TIMEOUT_STYLE_HIDDEN,
the user has asked GRUB to be quiet (for example to allow flickfree
boot) and thus the message should not be printed.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>