[libc] Redefine low 8 bits of error code as "platform error code"

The low 8 bits of an iPXE error code are currently defined as the
closest equivalent PXE error code.  Generalise this scheme to
platforms other than PC-BIOS by extending this definition to "closest
equivalent platform error code".  This allows for the possibility of
returning meaningful errors via EFI APIs.

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

src/util/einfo.c

@@ -38,10 +38,15 @@ struct options {
 
 /** Error usage information */
 struct einfo {
+	/** Size of error information record */
 	uint32_t size;
+	/** Error number */
 	uint32_t error;
+	/** Offset to error description (NUL-terminated) */
 	uint32_t desc;
+	/** Offset to file name (NUL-terminated) */
 	uint32_t file;
+	/** Line number */
 	uint32_t line;
 } __attribute__ (( packed ));