version: allow stating the coreboot revision in .coreboot-version

If .git doesn't exist, try to fetch the coreboot version from a file, before
falling back to a hard-code.

Change-Id: Idee8019c9a2b766fe69535367614c5254498335a
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10908
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)
diff --git a/.gitignore b/.gitignore
index f7447b4..770a62d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 .config
 .config.old
+.coreboot-version
 .xcompile
 .ccwrap
 build/