Google Link: Add remaining code to support native graphics

The Link native graphics commit 49428d84 [1]

    Add support for Google's Chromebook Pixel

was missing some of the higher level bits, and hence could not be
used.  This is not new code -- it has been working since last
August -- so the effort now is to get it into the tree and structure
it in a way compatible with upstream coreboot.

1. Add options to src/device/Kconfig to enable native graphics.
2. Export the MTRR function for setting variable MTRRs.
3. Clean up some of the comments and white space.

While I realize that the product name is Pixel, the mainboard in the
coreboot tree is called Link, and that name is what we will use
in our commits.

[1] http://review.coreboot.org/2482

Change-Id: Ie4db21f245cf5062fe3a8ee913d05dd79030e3e8
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/2531
Tested-by: build bot (Jenkins)

src/arch/x86/boot/coreboot_table.c

@@ -172,7 +172,7 @@ static void lb_console(struct lb_header *header)
 
 static void lb_framebuffer(struct lb_header *header)
 {
-#if CONFIG_FRAMEBUFFER_KEEP_VESA_MODE
+#if CONFIG_FRAMEBUFFER_KEEP_VESA_MODE || CONFIG_MAINBOARD_DO_NATIVE_VGA_INIT
 	void fill_lb_framebuffer(struct lb_framebuffer *framebuffer);
 	int vbe_mode_info_valid(void);