build system: Switch to fmap based firmware layout

We still add a master header for compatibility purposes, and the default
layouts don't cover anything non-coreboot (eg. IFD regions) yet.

The default layouts can be overridden by specifying an fmd file, from
which the fmap is generated.

Future work:
- map IFD regions to fmap regions
- non-x86: build minimalistic trampolines that jump into the first cbfs
  file, so the bootblock can be part of CBFS instead of reserving a
  whole 64K for it.
- teach coreboot's cbfs code to work without the master header
- teach coreboot's cbfs code to work on different fmap regions

Change-Id: Id1085dcd5107cf0e02e8dc1e77dc0dd9497a819c
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/11692
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/Kconfig

@@ -376,6 +376,14 @@ config CBFS_SIZE
 
 endmenu
 
+config FMDFILE
+	string "fmap description file in fmd format"
+	default ""
+	help
+	  The build system creates a default FMAP from ROM_SIZE and CBFS_SIZE,
+	  but in some cases more complex setups are required.
+	  When an fmd is specified, it overrides the default format.
+
 config SYSTEM_TYPE_LAPTOP
 	default n
 	bool