[spcr] Use the serial port defined by the ACPI SPCR by default

On platforms where we expect ACPI tables to exist, use the serial port
defined by the ACPI Serial Port Console Redirection (SPCR) table by
default, falling back to the fixed serial port defined at build time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/defaults/efi.h

diff --git a/src/config/defaults/efi.h b/src/config/defaults/efi.h
index ed7d37f..4c9ba9d 100644
--- a/src/config/defaults/efi.h
+++ b/src/config/defaults/efi.h
@@ -28,7 +28,7 @@
 #define FDT_EFI
 #define MPAPI_EFI
 #define NAP_EFI
-#define SERIAL_FIXED
+#define SERIAL_SPCR
 
 #define	NET_PROTO_IPV6		/* IPv6 protocol */
 #define	NET_PROTO_LLDP		/* Link Layer Discovery protocol */

src/config/defaults/pcbios.h

diff --git a/src/config/defaults/pcbios.h b/src/config/defaults/pcbios.h
index 9be9ff8..3f24f1b 100644
--- a/src/config/defaults/pcbios.h
+++ b/src/config/defaults/pcbios.h
@@ -26,7 +26,7 @@
 #define REBOOT_PCBIOS
 #define ACPI_RSDP
 #define MPAPI_PCBIOS
-#define SERIAL_FIXED
+#define SERIAL_SPCR
 
 #ifdef __x86_64__
 #define IOMAP_PAGES

src/config/serial.h

diff --git a/src/config/serial.h b/src/config/serial.h
index 6dc051d..a372963 100644
--- a/src/config/serial.h
+++ b/src/config/serial.h
@@ -26,6 +26,10 @@
 #define	COMSPEED	115200		/* Baud rate */
 #endif
 
+/* Uncomment these to ignore the ACPI SPCR table (if present) */
+//#undef SERIAL_SPCR
+//#define SERIAL_FIXED
+
 /* Early UART configuration (for bare metal prefix debugging only) */
 //#define EARLY_UART_MODEL	8250
 //#define EARLY_UART_REG_BASE	0x10000000