[build] Canonicalise settings sources configuration

Move all settings source selection from config/defaults/<platform>.h
to the top-level config/settings.h, using indented conditional blocks
to clarify which sources are supported and enabled on each platform.

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

Author: mcb30

src/config/defaults/efi.h

@@ -36,8 +36,6 @@ FILE_SECBOOT ( PERMITTED );
 #define	USB_EFI			/* Provide EFI_USB_IO_PROTOCOL interface */
 #define USB_BLOCK		/* USB block devices */
 
-#define EFI_SETTINGS		/* EFI variable settings */
-
 #if defined ( __i386__ ) || defined ( __x86_64__ )
 #define IOAPI_X86
 #define ENTROPY_RDRAND

src/config/settings.h

@@ -12,13 +12,27 @@ FILE_SECBOOT ( PERMITTED );
 
 #include <config/defaults.h>
 
-#define	PCI_SETTINGS	/* PCI device settings */
-#define	USB_SETTINGS	/* USB device settings */
-//#define	CPUID_SETTINGS	/* CPUID settings */
-//#define	MEMMAP_SETTINGS	/* Memory map settings */
-//#define	VMWARE_SETTINGS	/* VMware GuestInfo settings */
-//#define	VRAM_SETTINGS	/* Video RAM dump settings */
-//#define	ACPI_SETTINGS	/* ACPI settings */
+/* Settings sources supported on all platforms */
+//#define ACPI_SETTINGS		/* ACPI settings */
+#define PCI_SETTINGS		/* PCI device settings */
+#define USB_SETTINGS		/* USB device settings */
+
+/* Settings sources supported only on EFI platforms */
+#if defined ( PLATFORM_efi )
+  #define EFI_SETTINGS		/* EFI variable settings */
+#endif
+
+/* Settings sources supported only when memory maps are available */
+#if ! defined ( MEMMAP_NULL )
+  //#define MEMMAP_SETTINGS	/* Memory map settings */
+#endif
+
+/* Settings sources supported only on x86 CPUs */
+#if defined ( __i386__ ) || defined ( __x86_64__ )
+  //#define CPUID_SETTINGS	/* CPUID settings */
+  //#define VMWARE_SETTINGS	/* VMware GuestInfo settings */
+  //#define VRAM_SETTINGS	/* Video RAM dump settings */
+#endif
 
 #include <config/named.h>
 #include NAMED_CONFIG(settings.h)