add better source documentation for license compliance

SOURCE.md

@@ -0,0 +1,11 @@
+The images we release are the result of building coreboot using the
+configuration in our git repository at https://github.com/merge/skulls
+
+	git clone https://github.com/merge/skulls
+
+The coreboot source code is available via git:
+
+	git clone https://review.coreboot.org/coreboot.git
+
+In case there are external patches applied to build coreboot, they are
+available in the sources directory too.

x230/sources/PATCHES.md

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+This describes patches we apply to coreboot's master branch for building
+our release image:
+
+* Intel microcode update:
+
+	git pull https://review.coreboot.org/blobs refs/changes/63/25363/5