arches: lib: add main_decl.h for main() declaration

It is silly to have a single header to declare the main()
symbol, however some of the arches provided it while
lib/bootblock.c relied on the arch headers to declare it. Just
move the declaration into its own header file and utilize it.

Change-Id: I743b4c286956ae047c17fe46241b699feca73628
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13681
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

src/include/bootstate.h

@@ -20,6 +20,10 @@
 #include <stdlib.h>
 #include <stddef.h>
 #include <stdint.h>
+/* Only declare main() when in ramstage. */
+#if ENV_RAMSTAGE
+#include <main_decl.h>
+#endif
 
 /* Control debugging of the boot state machine. */
 #define BOOT_STATE_DEBUG 0
@@ -168,11 +172,6 @@ int boot_state_unblock(boot_state_t state, boot_state_sequence_t seq);
 void boot_state_current_block(void);
 void boot_state_current_unblock(void);
 
-#if ENV_RAMSTAGE
-/* Entry into the boot state machine. */
-void main(void);
-#endif
-
 /* In order to schedule boot state callbacks at compile-time specify the
  * entries in an array using the BOOT_STATE_INIT_ENTRIES and
  * BOOT_STATE_INIT_ENTRY macros below. */