cbfs: Add "struct" file type and associated helpers

This patch adds functionality to compile a C data structure into a raw
binary file, add it to CBFS and allow coreboot to load it at runtime.
This is useful in all cases where we need to be able to have several
larger data sets available in an image, but will only require a small
subset of them at boot (a classic example would be DRAM parameters) or
only require it in certain boot modes. This allows us to load less data
from flash and increase boot speed compared to solutions that compile
all data sets into a stage.

Each structure has to be defined in a separate .c file which contains no
functions and only a single global variable. The data type must be
serialization safe (composed of only fixed-width types, paying attention
to padding). It must be added to CBFS in a Makefile with the 'struct'
file processor.

Change-Id: Iab65c0b6ebea235089f741eaa8098743e54d6ccc
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/16272
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

payloads/libpayload/include/cbfs_core.h

@@ -73,6 +73,7 @@
 #define CBFS_TYPE_VSA        0x51
 #define CBFS_TYPE_MBI        0x52
 #define CBFS_TYPE_MICROCODE  0x53
+#define CBFS_TYPE_STRUCT     0x70
 #define CBFS_COMPONENT_CMOS_DEFAULT 0xaa
 #define CBFS_COMPONENT_CMOS_LAYOUT 0x01aa