add coverity cron job for weekly analysis

.github/travis/cron_build.sh

diff --git a/.github/travis/cron_build.sh b/.github/travis/cron_build.sh
new file mode 100755
index 0000000..30211d7
--- /dev/null
+++ b/.github/travis/cron_build.sh
@@ -0,0 +1,94 @@
+#!/bin/bash
+
+source .github/travis/common.sh
+set -e
+
+$SPACER
+
+if [ "_${_COVERITY_EMAIL}" == "_" ] ||
+[ "_${_COVERITY_URL}" == "_" ] ||
+[ "_${_COVERITY_MD5}" == "_" ]
+then
+  echo "_COVERITY_EMAIL, _COVERITY_URL and _COVERITY_MD5 must be set before using this script"
+  exit 1
+fi
+
+build_root=${PWD}
+
+##########################
+# download coverity tool
+start_section "coverity download" "${GREEN}Downloading coverity tool..${NC}"
+
+mkdir -p ${HOME}/coverity
+cd ${HOME}/coverity
+
+# get coverity and extract it
+wget --quiet ${_COVERITY_URL} --post-data="token=${COVERITY_SCAN_TOKEN}&project=Verilog+to+Routing" -O coverity.tar.gz
+echo "${_COVERITY_MD5} coverity.tar.gz" | md5sum -c -
+tar xzf coverity.tar.gz
+rm -f coverity.tar.gz
+
+# export the coverity tool
+coverity_dir=$(ls -d cov-analysis-linux64*)
+export PATH="${PATH}:${HOME}/coverity/${coverity_dir}/bin"
+which cov-build
+
+# make sure we use the compiler so we clear the cache, disable ccache and remove it from the path
+ccache --clear
+export CCACHE_DISABLE=1
+export PATH=${PATH/'/usr/bin/ccache:'/}
+export PATH=${PATH/':/usr/lib/ccache'/}
+
+#tell coverity about the CC compiler used
+cov-configure --template --compiler ${CC} --comptype gcc
+
+
+cd ${build_root}
+
+end_section "coverity download"
+
+
+$SPACER
+
+
+#######################
+# build with coverity
+start_section "coverity.vtr.build" "${GREEN}Building..${NC}"
+
+# make sure we use the compiler so we clear the cache
+
+# remove old files in case they exist (usefull if we run this locally)
+rm -f CMakeCache.txt
+rm -Rf build
+
+# prepare the build
+mkdir -p build
+cd build
+cmake -DCMAKE_BUILD_TYPE=debug -DVTR_ASSERT_LEVEL=3 -DWITH_BLIFEXPLORER=on ..
+
+# now build and scan
+cov-build --dir cov-int make -j$(nproc --all)
+
+end_section "coverity.vtr.build"
+
+
+$SPACER
+
+
+#######################
+# export to coverity
+start_section "coverity.export" "${GREEN}Exporting..${NC}"
+
+tar -czvf vtr_coverity.tar.gz cov-int
+
+curl --form token=${COVERITY_SCAN_TOKEN} \
+  --form email=${_COVERITY_EMAIL} \
+  --form file=@vtr_coverity.tar.gz\
+  --form version="master branch head" \
+  --form description="$(git log -1|head -1)" \
+  https://scan.coverity.com/builds?project=Verilog+to+Routing
+
+end_section "coverity.export"
+
+
+$SPACER

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index f8de94f..1349710 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -71,9 +71,14 @@
   - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
 
 stages:
-  #- Build
-  - Test
-  - Build Compatibility
+  # - name: Build
+  #   if: type != cron
+  - name: Test
+    if: type != cron
+  - name: Build Compatibility
+    if: type != cron
+  - name: Build Coverity
+    if: type = cron
 
 jobs:
   include:
@@ -160,6 +165,14 @@
          - MATRIX_EVAL="CC=clang-8 && CXX=clang++-8"
       script:
         - ./.github/travis/build.sh
+    - stage: Build Coverity
+      name: "Coverity Scan"
+      env:
+        - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
+        - _COVERITY_URL="https://scan.coverity.com/download/linux64"
+        - _COVERITY_MD5="d0d7d7df9d6609e578f85096a755fb8f"
+      script:
+        - ./.github/travis/cron_build.sh
 
 before_script:
   - source .github/travis/common.sh