buildsystem: Make CPU microcode updating more configurable

This patch aims to improve the microcode in CBFS handling that was
brought by the last patches from Stefan and the Chromium team.

Choices in Kconfig
  - 1) Generate microcode from tree (default)
  - 2) Include external microcode file
  - 3) Do not put microcode in CBFS

The idea is to give the user full control over including non-free
blobs in the final ROM image.

MICROCODE_INCLUDE_PATH Kconfig variable is eliminated. Microcode
is handled by a special class, cpu_microcode, as such:

cpu_microcode-y += microcode_file.c

MICROCODE_IN_CBFS should, in the future, be eliminated. Right now it is
needed by intel microcode updating. Once all intel cpus are converted to
cbfs updating, this variable can go away.

These files are then compiled and assembled into a binary CBFS file.
The advantage of doing it this way versus the current method is that
  1) The rule is CPU-agnostic
  2) Gives user more control over if and how to include microcode blobs
  3) The rules for building the microcode binary are kept in
   src/cpu/Makefile.inc, and thus would not clobber the other makefiles,
   which are already overloaded and very difficult to navigate.

Change-Id: I38d0c9851691aa112e93031860e94895857ebb76
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/1245
Tested-by: build bot (Jenkins)
Reviewed-by: Anton Kochkov <anton.kochkov@gmail.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/cpu/intel/microcode/microcode.c

@@ -28,7 +28,7 @@
 #include <cpu/x86/msr.h>
 #include <cpu/intel/microcode.h>
 
-#if CONFIG_MICROCODE_IN_CBFS
+#if CONFIG_CPU_MICROCODE_IN_CBFS
 #ifdef __PRE_RAM__
 #include <arch/cbfs.h>
 #else
@@ -77,7 +77,7 @@ static inline u32 read_microcode_rev(void)
 	return msr.hi;
 }
 
-#if CONFIG_MICROCODE_IN_CBFS
+#if CONFIG_CPU_MICROCODE_IN_CBFS
 static
 #endif
 void intel_update_microcode(const void *microcode_updates)
@@ -144,9 +144,9 @@ void intel_update_microcode(const void *microcode_updates)
 	}
 }
 
-#if CONFIG_MICROCODE_IN_CBFS
+#if CONFIG_CPU_MICROCODE_IN_CBFS
 
-#define MICROCODE_CBFS_FILE "microcode_blob.bin"
+#define MICROCODE_CBFS_FILE "cpu_microcode_blob.bin"
 
 void intel_update_microcode_from_cbfs(void)
 {