[coreboot] LZMA magic number

[Myles Watson]

Is there a magic number for LZMA?  Should there be?

I accidentally used an uncompressed payload with v2 when it expected a
compressed payload, and it gave me the message:

Decoder scratchpad too small!!
       Decoding error = 1

I think it would be nicer to have an error like:

Payload not compressed with lzma!

Myles