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/include/cpu/x86/mtrr.h

@@ -40,6 +40,13 @@
 
 #if !defined (__ASSEMBLER__) && !defined(__PRE_RAM__)
 #include <device/device.h>
+/* You should almost NEVER use this function.
+ * N.B. We worked on a lot of ways to make this continue as static,
+ * but just making it available ended up being the simplest solution.
+ */
+void set_var_mtrr(
+	unsigned int reg, unsigned long basek, unsigned long sizek,
+	unsigned char type, unsigned address_bits);
 void enable_fixed_mtrr(void);
 void x86_setup_var_mtrrs(unsigned int address_bits, unsigned int above4gb);
 void x86_setup_mtrrs(void);