meson: Determine custom_baud source file only once

Change-Id: I13221bdca7d14a483f416e81e3830a495659a85e
Signed-off-by: Peter Stuge <peter@stuge.se>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/70568
Original-Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Original-Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73478
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
1 file changed