cpu/microcode: Remove EXTERNAL / ADDED_DURING_BUILD variables

There has been a concerted effort to clean up coreboot's microcode
handling that has included a move away from coreboot-specific
microcode file collections.  As a result, the ability to specify
a single microcode file to be added to the image is of less utility
than before.

NOTE: This patch remove the built-in external microcode feature,
however the user can still specify no microcode during build and
manually add the correct microcode file(s) to the CBFS image after
the build is complete.

Change-Id: Ifea94c21e531a74953f5a0e2f489378c20ef3b5c
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: http://review.coreboot.org/11903
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)
diff --git a/Makefile.inc b/Makefile.inc
index feadd0d..de148cb 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -634,7 +634,7 @@
 	$(CBFSTOOL) $@.tmp add -f $(CONFIG_PXE_ROM_FILE) -n pci$(CONFIG_PXE_ROM_ID).rom -t raw
 	@printf "    UPDATE-FIT \n"
 	$(CBFSTOOL) $@.tmp update-fit -n cpu_microcode_blob.bin -x $(CONFIG_CPU_INTEL_NUM_FIT_ENTRIES)