bzip2/lzma: update boot protocol specification

Impact: documentation

Update the boot protocol specification to include the currently
supported file formats and their magic numbers.
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>

Documentation/x86/boot.txt

@@ -542,7 +542,10 @@ Protocol:	2.08+
 
   The payload may be compressed. The format of both the compressed and
   uncompressed data should be determined using the standard magic
-  numbers. Currently only gzip compressed ELF is used.
+  numbers.  The currently supported compression formats are gzip
+  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A) and LZMA
+  (magic number 5D 00).  The uncompressed payload is currently always ELF
+  (magic number 7F 45 4C 46).
   
 Field name:	payload_length
 Type:		read