efi: Explain OVMF acronym in OVMF_DEBUG_LOG help text

People not very intimate with EFI may not know the meaning of the OVMF
acronym.  Write it in full, to help users with making good decisions
when configuring their kernels.

Fixes: f393a761 ("efi: add ovmf debug log driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Richard Lyu <richard.lyu@suse.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>