meson: Build the classic cli / flashrom cli executable as feature

Provide the meson option `-Dclassic_cli=auto/enabled/disabled` to
determine if the classic cli flashrom executable should be built. This
is needed to support targets, like libpayload, which can only build
libflashrom.

Change-Id: If39981ad0b3281edd6fc7e220362d2938dddf814
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/66705
Original-Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Original-Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72355
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
2 files changed