x86: make bootblock size for C_ENVIRONMENT_BOOTBLOCK configurable

Certain platforms may need to limit their bootblock size to within
a given size because specific constraints. Allow the size to be
provided by the mainboard or chipset by way of the arch Kconfig
being processed after those.

Change-Id: I46cc6315918cde575070fa2d3e2514f28008f575
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13691
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>

src/arch/x86/Kconfig

@@ -146,3 +146,8 @@ config ID_SECTION_OFFSET
 config COMPILE_IN_DSDT
 	bool "compile in DSDT and use that over DSDT in CBFS"
 	default n
+
+# 64KiB default bootblock size when employing C_ENVIRONMENT_BOOTBLOCK.
+config C_ENV_BOOTBLOCK_SIZE
+	hex
+	default 0x10000

src/arch/x86/memlayout.ld

@@ -45,7 +45,8 @@ SECTIONS
 #elif ENV_BOOTBLOCK
 	/* This is for C_ENVIRONMENT_BOOTBLOCK. arch/x86/bootblock.ld contains
 	 * the logic for the romcc linking. */
-	BOOTBLOCK(0xffff0000, 64K)
+	BOOTBLOCK(0xffffffff - CONFIG_C_ENV_BOOTBLOCK_SIZE + 1,
+			CONFIG_C_ENV_BOOTBLOCK_SIZE)
 
 	/* Pull in the cache-as-ram rules. */
 	#include "car.ld"