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/bootblock_common.h

@@ -16,6 +16,8 @@
 #ifndef __BOOTBLOCK_COMMON_H
 #define __BOOTBLOCK_COMMON_H
 
+#include <main_decl.h>
+
 /* These are defined as weak no-ops that can be overridden by mainboard/SoC. */
 void bootblock_mainboard_early_init(void);
 void bootblock_mainboard_init(void);