Clean up stack checking code

Several small improvements of the stack checking code:
- move the CPU0 stack check right before jumping to the payload
  and out of hardwaremain (that file is too crowded anyways)
- fix prototype in lib.h
- print size of used stack
- use checkstack function both on CPU0 and CPU1-x
- print amount of stack used per core

Signed-off-by: Stefan Reinauer <reinauer@google.com>

Test: Boot coreboot on Link, see the following output:
     ...
     CPU1: stack: 00156000 - 00157000, lowest used address 00156c68,
           stack used: 920 bytes
     CPU2: stack: 00155000 - 00156000, lowest used address 00155c68,
           stack used: 920 bytes
     CPU3: stack: 00154000 - 00155000, lowest used address 00154c68,
           stack used: 920 bytes
     ...
     Jumping to boot code at 1110008
     CPU0: stack: 00157000 - 00158000, lowest used address 00157af8,
           stack used: 1288 bytes

Change-Id: I7b83eeee0186559a0a62daa12e3f7782990fd2df
Reviewed-on: http://review.coreboot.org/1787
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/lib.h

@@ -41,10 +41,10 @@ int ram_check_nodie(unsigned long start, unsigned long stop);
 void quick_ram_check(void);
 
 /* Defined in src/lib/stack.c */
-int checkstack(void *top_of_stack, int stacksize);
+int checkstack(void *top_of_stack, int core);
 
 /* currently defined by a ldscript */
-extern u8 _estack;
+extern unsigned char _estack[];
 
 /* Defined in romstage.c */
 #if CONFIG_CPU_AMD_GEODE_LX