arm: Add support for a preram_cbmem_console symbol.

This symbol is set using a config variable which can be set to something
appropriate by the SOC. If it isn't, the symbol is set to 0 which should be
caught by checks in the cbmem console itself.

BUG=None
TEST=Built for nyan with a cbmem buffer location set. Built for peach_pit
without a location set.
BRANCH=None

Change-Id: I92cd65bb6767a67637faf1dd3cdbe03e433724a9
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://chromium-review.googlesource.com/193165
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>

src/arch/arm/Kconfig

@@ -36,6 +36,10 @@ config ARM_BOOTBLOCK_NORMAL
 
 endchoice
 
+config CBMEM_CONSOLE_PRERAM_BASE
+	hex
+	depends on CONSOLE_CBMEM
+
 config CPU_HAS_BOOTBLOCK_INIT
 	bool
 	default n

src/arch/arm/bootblock.ld

@@ -49,6 +49,8 @@ SECTIONS
 		*(.sbss.*);
 	} : to_load = 0xff
 
+	preram_cbmem_console = CONFIG_CBMEM_CONSOLE_PRERAM_BASE;
+
 	/DISCARD/ : {
 		*(.comment)
 		*(.note)

src/arch/arm/romstage.ld

@@ -72,6 +72,8 @@ SECTIONS
 
 	_end = .;
 
+	preram_cbmem_console = CONFIG_CBMEM_CONSOLE_PRERAM_BASE;
+
 	/* Discard the sections we don't need/want */
 	/DISCARD/ : {
 		*(.comment)