include/fmap.h: Require FMAP_FLASH_SIZE == CONFIG_ROM_SIZE

In our current tree this is always the case. The coreboot code (as far
as I know) doesn't have a hard requirement on that. But if these values
differ then it is usually always a mistake made by the programmer and it
is hard to catch since the value don't really depend on each other.
So until a time in which there comes a platform which needs a flashmap
that doesn't cover the whole flash, this check is introduced.

For that purpose we need to replace the default .config file for tests,
because otherwise the check won't match.

The config file that is used now is based on the fact that we use the
same mainboard for the `.config` as for the `fmap_config.h` in
`tests/include/tests/lib/fmap/fmap_config.h`

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I8bc05a17a2630516ede949660b4fc428f199f3ab
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90264
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

tests/include/tests/lib/fmap_config.h

@@ -8,5 +8,6 @@
  * Keeps the build tests from failing since fmap.h needs fmap_config.h
  */
 #define FMAP_SECTION_FLASH_START 0
+#define FMAP_SECTION_FLASH_SIZE 0x1000000
 
 #endif