[settings] Expose build architecture and platform via settings

Expose the build architecture (e.g. "i386" or "x86_64") via
${buildarch} and the firmware platform (e.g. "pcbios" or "efi") via
${platform}.  These settings directly expose the ARCH and PLATFORM
variables from the Makefile.

Note that the build architecture reflects the architecture for which
iPXE was compiled, not the architecture on which iPXE is currently
running.  The "cpuid" command can be used to detect a 64-bit system at
runtime.

Requested-by: James A. Peltier <jpeltier@sfu.ca>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

  ******************************************************************************
  */
 
+/** A built-in setting operation */
+struct builtin_setting_operation {
+	/** Setting */
+	struct setting *setting;
+	/** Fetch setting value
+	 *
+	 * @v data		Buffer to fill with setting data
+	 * @v len		Length of buffer
+	 * @ret len		Length of setting data, or negative error
+	 */
+	int ( * fetch ) ( void *data, size_t len );
+};
+
 /** Built-in setting tag magic */
 #define BUILTIN_SETTING_TAG_MAGIC 0xb1
 
 /**
  * Fetch error number setting
  *
- * @v settings		Settings block
- * @v setting		Setting to fetch
- * @v data		Setting data, or NULL to clear setting
- * @v len		Length of setting data
- * @ret rc		Return status code
+ * @v data		Buffer to fill with setting data
+ * @v len		Length of buffer
+ * @ret len		Length of setting data, or negative error
  */
-static int errno_fetch ( struct settings *settings __unused,
-			 struct setting *setting __unused,
-			 void *data, size_t len ) {
+static int errno_fetch ( void *data, size_t len ) {
 	uint32_t content;
 
 	/* Return current error */
 	return sizeof ( content );
 }
 
+/** "buildarch" setting tag */
+#define BUILTIN_SETTING_TAG_BUILDARCH BUILTIN_SETTING_TAG ( 0x02 )
+
+/** Build architecture setting */
+struct setting buildarch_setting __setting ( SETTING_MISC ) = {
+	.name = "buildarch",
+	.description = "Build architecture",
+	.tag = BUILTIN_SETTING_TAG_BUILDARCH,
+	.type = &setting_type_string,
+};
+
+/**
+ * Fetch build architecture setting
+ *
+ * @v data		Buffer to fill with setting data
+ * @v len		Length of buffer
+ * @ret len		Length of setting data, or negative error
+ */
+static int buildarch_fetch ( void *data, size_t len ) {
+	static const char buildarch[] = _S2 ( ARCH );
+
+	strncpy ( data, buildarch, len );
+	return ( sizeof ( buildarch ) - 1 /* NUL */ );
+}
+
+/** "platform" setting tag */
+#define BUILTIN_SETTING_TAG_PLATFORM BUILTIN_SETTING_TAG ( 0x03 )
+
+/** Platform setting */
+struct setting platform_setting __setting ( SETTING_MISC ) = {
+	.name = "platform",
+	.description = "Platform",
+	.tag = BUILTIN_SETTING_TAG_PLATFORM,
+	.type = &setting_type_string,
+};
+
+/**
+ * Fetch platform setting
+ *
+ * @v data		Buffer to fill with setting data
+ * @v len		Length of buffer
+ * @ret len		Length of setting data, or negative error
+ */
+static int platform_fetch ( void *data, size_t len ) {
+	static const char platform[] = _S2 ( PLATFORM );
+
+	strncpy ( data, platform, len );
+	return ( sizeof ( platform ) - 1 /* NUL */ );
+}
+
+/** List of built-in setting operations */
+static struct builtin_setting_operation builtin_setting_operations[] = {
+	{ &errno_setting, errno_fetch },
+	{ &buildarch_setting, buildarch_fetch },
+	{ &platform_setting, platform_fetch },
+};
+
 /**
  * Fetch built-in setting
  *
  * @v settings		Settings block
  * @v setting		Setting to fetch
- * @v data		Setting data, or NULL to clear setting
- * @v len		Length of setting data
- * @ret rc		Return status code
+ * @v data		Buffer to fill with setting data
+ * @v len		Length of buffer
+ * @ret len		Length of setting data, or negative error
  */
 static int builtin_fetch ( struct settings *settings __unused,
 			   struct setting *setting,
 			   void *data, size_t len ) {
+	struct builtin_setting_operation *builtin;
+	unsigned int i;
 
-	if ( setting_cmp ( setting, &errno_setting ) == 0 ) {
-		return errno_fetch ( settings, setting, data, len );
-	} else {
-		return -ENOENT;
+	for ( i = 0 ; i < ( sizeof ( builtin_setting_operations ) /
+			    sizeof ( builtin_setting_operations[0] ) ) ; i++ ) {
+		builtin = &builtin_setting_operations[i];
+		if ( setting_cmp ( setting, builtin->setting ) == 0 )
+			return builtin->fetch ( data, len );
 	}
+	return -ENOENT;
 }
 
 /**