lc15: use environment for firmware version

This allows to specify firmware version to use by jenkins builder via
environment variable. If environment variable does not exist than
default master branch is used.

Change-Id: If8b249aa00270e60a0449f089a16823976e54d54
Related: SYS#3683
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 9c5c2e2..5222476 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -32,7 +32,7 @@
 elif [ "$with_dsp" = lc15 ]; then
   PCU_CONFIG="$PCU_CONFIG --enable-lc15bts-phy --with-litecell15=$deps/layer1-headers/inc"
   cd "$deps"
-  osmo-layer1-headers.sh lc15
+  osmo-layer1-headers.sh lc15 "$FIRMWARE_VERSION"
   cd "$base"
 
 elif [ -z "$with_dsp" -o "$with_dsp" = none ]; then