cbfs: Add LZ4 in-place decompression support for pre-RAM stages

This patch ports the LZ4 decompression code that debuted in libpayload
last year to coreboot for use in CBFS stages (upgrading the base
algorithm to LZ4's dev branch to access the new in-place decompression
checks). This is especially useful for pre-RAM stages in constrained
SRAM-based systems, which previously could not be compressed due to
the size requirements of the LZMA scratchpad and bounce buffer. The
LZ4 algorithm offers a very lean decompressor function and in-place
decompression support to achieve roughly the same boot speed gains
(trading compression ratio for decompression time) with nearly no
memory overhead.

For now we only activate it for the stages that had previously not been
compressed at all on non-XIP (read: non-x86) boards. In the future we
may also consider replacing LZMA completely for certain boards, since
which algorithm wins out on boot speed depends on board-specific
parameters (architecture, processor speed, SPI transfer rate, etc.).

BRANCH=None
BUG=None
TEST=Built and booted Oak, Jerry, Nyan and Falco. Measured boot time on
Oak to be about ~20ms faster (cutting load times for affected stages
almost in half).

Change-Id: Iec256c0e6d585d1b69985461939884a54e3ab900
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/13638
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/Kconfig

@@ -156,6 +156,17 @@ config COMPRESS_RAMSTAGE
 	  that decompression might slow down booting if the boot flash
 	  is connected through a slow link (i.e. SPI).
 
+config COMPRESS_PRERAM_STAGES
+	bool "Compress romstage and verstage with LZ4"
+	default y if !ARCH_X86
+	default n
+	help
+	  Compress romstage and (if it exists) verstage with LZ4 to save flash
+	  space and speed up boot, since the time for reading the image from SPI
+	  (and in the vboot case verifying it) is usually much greater than the
+	  time spent decompressing. Doesn't work for XIP stages (assume all
+	  ARCH_X86 for now) for obvious reasons.
+
 config INCLUDE_CONFIG_FILE
 	bool "Include the coreboot .config file into the ROM image"
 	default y