libpayload: Split off generic serial API from 8250 driver

There is a lot of generic code in the 8250 driver that should
be available for non-8250 systems with serial ports as well.

Change-Id: I67fcb12b5fa99ae0047b3cbf1815043d3919437e
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/14371
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/payloads/libpayload/drivers/Makefile.inc b/payloads/libpayload/drivers/Makefile.inc
index 31eb43c..644b0d9 100644
--- a/payloads/libpayload/drivers/Makefile.inc
+++ b/payloads/libpayload/drivers/Makefile.inc
@@ -33,10 +33,10 @@
 
 libc-$(CONFIG_LP_SPEAKER) += speaker.c
 
-libc-$(CONFIG_LP_8250_SERIAL_CONSOLE) += serial/8250.c
-libc-$(CONFIG_LP_S5P_SERIAL_CONSOLE) += serial/s5p.c
-libc-$(CONFIG_LP_IPQ806X_SERIAL_CONSOLE) += serial/ipq806x.c
-libc-$(CONFIG_LP_BG4CD_SERIAL_CONSOLE) += serial/bg4cd.c
+libc-$(CONFIG_LP_8250_SERIAL_CONSOLE) += serial/8250.c serial/serial.c
+libc-$(CONFIG_LP_S5P_SERIAL_CONSOLE) += serial/s5p.c serial/serial.c
+libc-$(CONFIG_LP_IPQ806X_SERIAL_CONSOLE) += serial/ipq806x.c serial/serial.c
+libc-$(CONFIG_LP_BG4CD_SERIAL_CONSOLE) += serial/bg4cd.c serial/serial.c
 libc-$(CONFIG_LP_PC_KEYBOARD) += keyboard.c
 
 libc-$(CONFIG_LP_CBMEM_CONSOLE) += cbmem_console.c