include/fmap.h: Require FMAP_SECTION_FLASH_START == 0

For simplicity we are going to impose this restriction to coreboot.
Note however that this is only a restriction for coreboot itself. The
FMAP tool itself is still a generic tool that does not require the FMAP
to start at offset 0.

Add an defacto empty fmap_config.h to our test cases, since fmap.h now
includes fmap_config.h.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: Iba04ebdcd5557664a865d2854028dd811f052249
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86770
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>

src/include/fmap.h

@@ -5,9 +5,14 @@
 
 #include <commonlib/bsd/fmap_serialized.h>
 #include <commonlib/region.h>
+#include <fmap_config.h>
 #include <stddef.h>
 #include <stdint.h>
 
+#if FMAP_SECTION_FLASH_START != 0
+	#error "FMAP must always start flash address 0"
+#endif
+
 /* Locate the named area in the fmap and fill in a region device representing
  * that area. The region is a sub-region of the readonly boot media. Return
  * 0 on success, < 0 on error. */