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/cpu/x86/mtrr/mtrr.c

@@ -83,7 +83,7 @@ static void enable_var_mtrr(void)
 }
 
 /* setting variable mtrr, comes from linux kernel source */
-static void set_var_mtrr(
+void set_var_mtrr(
 	unsigned int reg, unsigned long basek, unsigned long sizek,
 	unsigned char type, unsigned address_bits)
 {