firmware/Makefile: fix UNKNOWN in OBS packages

Fix having the version set to UNKNOWN in all packages built by OBS. The
osmo-ci.git scripts generating the source packages to be built by OBS
generate a ".tarball-version" file with a version string like
"0.7.0.70-657c", but it did not get used because the path was wrong.

Related: OS#4413
Related: https://osmocom.org/projects/cellular-infrastructure/wiki/Git-version-gen
Change-Id: Ic0b06011a604beec7c1c907c2c6e4ae927456e2e
diff --git a/firmware/Makefile b/firmware/Makefile
index 7be5c30..d7c64b9 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -28,7 +28,7 @@
 
 #   Makefile for compiling the Getting Started with SAM3S Microcontrollers project
 
-GIT_VERSION=$(shell $(TOP)/git-version-gen $(TOP)/.tarvers)
+GIT_VERSION=$(shell $(TOP)/git-version-gen $(TOP)/.tarball-version)
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