microblaze: Clean microblaze initialization

Move board specific function to board_init function in board/ folder
Remove externs from generic board.c
Use board_init_f function in board.c file.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Acked-by: Stephan Linz <linz@li-pro.net>

arch/microblaze/include/asm/processor.h

@@ -28,4 +28,7 @@
 extern char __end[];
 extern char __text_start[];
 
+/* Microblaze board initialization function */
+void board_init(void);
+
 #endif /* __ASM_MICROBLAZE_PROCESSOR_H */