payloads/iPXE: add iPXE serial enable option

Since SeaBIOS 1.11.0 implements serial console and etc/sercon-port
runtime config file is present in CBFS, SeaBIOS additionally
redirects iPXE output to configured IO port. For boards which use
the same UART for SeaBIOS and iPXE console it causes doubled
output.

The option is enabled by default and passes UART configuration
to iPXE Makefile as before. When unselected, only SeaBIOS handles
printing output from iPXE.

Change-Id: Ia3c74cfbee4f816782161fcff286a14f46be78c5
Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Signed-off-by: Piotr Król <piotr.krol@3mdeb.com>
Reviewed-on: https://review.coreboot.org/27302
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
2 files changed