[serial] Allow platform to specify mechanism for identifying console

Allow the platform configuration to provide a mechanism for
identifying the serial console UART.  Provide two globally available
mechanisms: "null" (i.e. no serial console), and "fixed" (i.e. use
whatever is specified by COMCONSOLE in config/serial.h).

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

src/config/defaults/efi.h

@@ -28,6 +28,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define FDT_EFI
 #define MPAPI_EFI
 #define NAP_EFI
+#define SERIAL_FIXED
 
 #define	NET_PROTO_IPV6		/* IPv6 protocol */
 #define	NET_PROTO_LLDP		/* Link Layer Discovery protocol */

src/config/defaults/linux.h

@@ -24,6 +24,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #define DMAAPI_FLAT
 #define ACPI_LINUX
 #define MPAPI_NULL
+#define SERIAL_NULL
 
 #define DRIVERS_LINUX
 

src/config/defaults/pcbios.h

@@ -26,6 +26,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define REBOOT_PCBIOS
 #define ACPI_RSDP
 #define MPAPI_PCBIOS
+#define SERIAL_FIXED
 
 #ifdef __x86_64__
 #define IOMAP_PAGES

src/config/defaults/sbi.h

@@ -25,6 +25,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define REBOOT_SBI
 #define UMALLOC_UHEAP
 #define MEMMAP_FDT
+#define SERIAL_NULL
 
 #define ACPI_NULL
 #define MPAPI_NULL

src/config/serial.h

@@ -13,6 +13,8 @@
 
 FILE_LICENCE ( GPL2_OR_LATER );
 
+#include <config/defaults.h>
+
 #define	COMCONSOLE	COM1		/* I/O port address */
 
 /* Keep settings from a previous user of the serial port (e.g. lilo or