libpayload: only compile drivers/serial.c on machines that use it.

Create a new serial console variable, X86_SERIAL_CONSOLE
which is only enabled when SERIAL_CONSOLE and ARCH_X86 are defined.

Builds for x86 and ARM.

Change-Id: I607253c418de015975a839e3c33577842885ec0c
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/2412
Tested-by: build bot (Jenkins)
Reviewed-by: Gabe Black <gabeblack@chromium.org>

payloads/libpayload/Config.in

@@ -168,9 +168,14 @@ config SERIAL_CONSOLE
 	bool "See output on the serial port console"
 	default y
 
+config X86_SERIAL_CONSOLE
+	bool
+	depends on ARCH_X86 && SERIAL_CONSOLE
+	default y
+
 config SERIAL_IOBASE
 	hex "I/O base for the serial port (default 0x3f8)"
-	depends on SERIAL_CONSOLE
+	depends on X86_SERIAL_CONSOLE
 	default 0x3f8
 
 config SERIAL_SET_SPEED

payloads/libpayload/drivers/Makefile.inc

@@ -33,7 +33,7 @@ libc-$(CONFIG_PCI) += pci.c
 
 libc-$(CONFIG_SPEAKER) += speaker.c
 
-libc-$(CONFIG_SERIAL_CONSOLE) += serial.c
+libc-$(CONFIG_X86_SERIAL_CONSOLE) += serial.c
 
 libc-$(CONFIG_PC_KEYBOARD) += keyboard.c