pc80: Move set_boot_successful()

Don't implement arch or driver -specific code under lib/,

Change-Id: If75980ec5efc622582e2b5e124ad0e7ee3fa39a3
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/17793
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/include/fallback.h

@@ -4,6 +4,7 @@
 #if !defined(__ASSEMBLER__)
 
 void boot_successful(void);
+void set_boot_successful(void);
 
 #endif /* __ASSEMBLER__ */