Reduce default stack size to 4K

coreboot uses about 2K of stack on the BSP, and about 1K of stack on the
APs. No reason to use an overdimensonal stack of 32k per core/thread.

Change-Id: I734c240b992d40e1e35db3df5437c36da0a755cf
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1780
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/x86/Kconfig

@@ -25,7 +25,7 @@ config RAMTOP
 
 config STACK_SIZE
 	hex
-	default 0x8000
+	default 0x1000
 
 # Maximum reboot count
 # TODO: Improve description.