armv7/exynos5250: fix usage of _stack and _estack

This patch fixes up the usage of stack pointer and regions.
The current approach only works by coincidence, so this fixes a few
things at once to get it into a working state and allow us to use
checkstack() again:

- Add a STACK_SIZE Kconfig variable. Earlier on it was evaluated to 0.

- Assign _stack and _estack using CPU-specific Kconfig variables since
  it may reside elsewhere in memory (not necessarily DRAM).

- Make the existing IRAM stack variables more useful in this context.

Change-Id: I4ca5b5680c9ea7e26b1b2b6b3890e028188b51c2
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/2416
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)

src/arch/armv7/bootblock.inc

@@ -94,4 +94,4 @@ wait_for_interrupt:
  */
 .align 2
 .Stack:
-	.word CONFIG_IRAM_STACK
+	.word CONFIG_STACK_TOP

src/arch/armv7/coreboot_ram.ld

@@ -99,15 +99,6 @@ SECTIONS
 	 * this line.
 	 */
 
-	. = ALIGN(CONFIG_STACK_SIZE);
-
-	_stack = .;
-	.stack . : {
-		/* Reserve a stack for each possible cpu */
-		. += CONFIG_MAX_CPUS*CONFIG_STACK_SIZE;
-	}
-	_estack = .;
-
         _heap = .;
         .heap . : {
                 /* Reserve CONFIG_HEAP_SIZE bytes for the heap */
@@ -116,6 +107,9 @@ SECTIONS
         }
         _eheap = .;
 
+	_stack = CONFIG_STACK_BOTTOM;
+	_estack = CONFIG_STACK_TOP;
+
 	/* The ram segment. This includes all memory used by the memory
 	 * resident copy of coreboot, except the tables that are produced on
 	 * the fly, but including stack and heap.