The code in lzma_literal() checks to see if we are at the start of the
compressed input data in order to determine whether or not a most
recent output byte exists.  This check is incorrect, since
initialisation of the decompressor will always consume the first five
bytes of the compressed input data.

Fix by instead checking whether or not we are at the start of the
output data stream.  This is, in any case, a more logical check.

This issue was masked during development and testing since virtual
machines tend to zero the initial contents of RAM; the spuriously-read
"most recent output byte" is therefore likely to already be a zero
when running in a virtual machine.

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

The 0xe9 debug port exists only on virtual machines.  Provide an
option to print debug output on the BIOS console, to allow for
debugging on real hardware.

Note that this option can be used only if the decompressor is called
in flat real mode; the easiest way to achieve this is to build with
DEBUG=libprefix.

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

LZMA performs an extra normalisation after decompression is complete,
which does not affect the output but may consume an extra byte from
the input (and so may affect which byte is identified as being the
start of the next block).

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

LZMA provides significantly better compression (by ~15%) than the
current NRV2B algorithm.

We use a raw LZMA stream (aka LZMA1) to avoid the need for code to
parse the LZMA2 block headers.  We use parameters {lc=2,lp=0,pb=0} to
reduce the stack space required by the decompressor to acceptable
levels (around 8kB).  Using lc=3 or pb=2 would give marginally better
compression, but at the cost of substantially increasing the required
stack space.

The build process now requires the liblzma headers to be present on
the build system, since we do not include a copy of an LZMA compressor
within the iPXE source tree.  The decompressor is written from scratch
(based on XZ Embedded) and is entirely self-contained within the
iPXE source.

The branch-call-jump (BCJ) filter used to improve the compressibility
is specific to iPXE.  We choose not to use liblzma's built-in BCJ
filter since the algorithm is complex and undocumented.  Our BCJ
filter achieves approximately the same results (on typical iPXE
binaries) with a substantially simpler algorithm.

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