bzip2/lzma: centralize format detection

Centralize the compression format detection to a common routine in the
lib directory, and use it for both initramfs and initrd.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

include/linux/decompress/generic.h

@@ -26,5 +26,8 @@ typedef int (*decompress_fn) (unsigned char *inbuf, int len,
  *fill should be called (repeatedly...) to read data, at most IOBUF_SIZE
  */
 
+/* Utility routine to detect the decompression method */
+decompress_fn decompress_method(const unsigned char *inbuf, int len,
+				const char **name);
 
 #endif