Update Travis CI

Add support for Travis CI cron jobs
Enable ccache
Renamed internal variables.
Use environment variable for arch instead of fixed value.
Fetch stable and mainline kernel version dynamically.
Removed manual jobs and use Travis matrix expansion.
Updated GCC to 7.5

.travis.yml

@@ -4,62 +4,52 @@ dist: bionic
 os: linux
 
 before_install:
-  - export KERNEL_URL_DETAILS=$(wget --quiet -O - ${KERNEL_URL}v${KVER}/ | grep -A8 'Build for amd64')
+  #Cron builds only build mainline kernel. Stable and LTS kernels usually not have breaking changes.
+  - if [ "$TRAVIS_EVENT_TYPE" == "cron" ] && [ "$KVER_BUILD" != "$KERNEL_MAINLINE" ]; then exit 0; fi
+  - export KERNEL_URL_DETAILS=$(wget --quiet -O - ${KERNEL_URL}v${KVER_BUILD}/ | grep -A8 "Build for ${TRAVIS_CPU_ARCH}")
   - export ALL_DEB=$(echo "$KERNEL_URL_DETAILS" |  grep -m1 'all.deb' | cut -d '"' -f 2)
-  - export KVER_BUILD=$(echo $ALL_DEB | cut -d '_' -f 1 | cut -c15-)
-  - wget ${KERNEL_URL}v${KVER}/$(echo "$KERNEL_URL_DETAILS" | grep -m1 'amd64.deb' | cut -d '"' -f 2)
-  - wget ${KERNEL_URL}v${KVER}/$ALL_DEB
+  - export KVER=$(echo $ALL_DEB | cut -d '_' -f 1 | cut -c15-)-generic
+  - wget ${KERNEL_URL}v${KVER_BUILD}/$(echo "$KERNEL_URL_DETAILS" | grep -m1 "${TRAVIS_CPU_ARCH}.deb" | cut -d '"' -f 2)
+  - wget ${KERNEL_URL}v${KVER_BUILD}/$ALL_DEB
   - sudo dpkg -i *.deb
 
-script:
-  - make CC=$COMPILER KVER=$KVER_BUILD-generic
+script: make CC=$CC KVER=$KVER
+
+addons:
+  apt:
+    packages:
+      #Force update to GCC-7.5 in order to compile Kernels >= 5.4.
+      - gcc-7
 env:
   global:
     - KERNEL_URL=https://kernel.ubuntu.com/~kernel-ppa/mainline/
+    - KERNEL_MAINLINE=$(curl -s https://www.kernel.org/releases.json | grep -B1 'mainline' | head -1 | cut -d'"' -f4)
+    - KERNEL_STABLE=$(curl -s https://www.kernel.org/releases.json | grep -A1 'latest_stable' | tail -1 | cut -d'"' -f4)
+  jobs:
+    - KVER_BUILD=$KERNEL_MAINLINE
+    - KVER_BUILD=$KERNEL_STABLE
+    - KVER_BUILD=5.5.19 #EOL
+ #Kernels 5.4 with minor versions > 28 are failing on amd64. Not upgrade the minor version without check https://kernel.ubuntu.com/~kernel-ppa/mainline/ 
+    - KVER_BUILD=5.4.28
+    - KVER_BUILD=4.19.116
+    - KVER_BUILD=4.9.219
+    - KVER_BUILD=4.4.219
+    - KVER_BUILD=3.16.82
+cache:
+  - ccache: true
 
 jobs:
+#Mainline kernel is also compiled on GCC 8 & 9. Jobs are added to the build matrix expansion
   include:
-    - compiler: gcc
-      addons:
+    - addons:
         apt:
           sources:
             - sourceline: "ppa:ubuntu-toolchain-r/test"
           packages:
             - gcc-9
-      env: COMPILER=gcc-9 KVER="$(curl -s https://www.kernel.org/ | grep -A1 'mainline:' | grep -oP '(?<=strong>).*(?=</strong.*)')"
-    - compiler: gcc
-      addons:
+      env: CC=gcc-9 KVER_BUILD=$KERNEL_MAINLINE
+    - addons:
         apt:
           packages:
             - gcc-8
-      env: COMPILER=gcc-8 KVER="$(curl -s https://www.kernel.org/ | grep -A1 'mainline:' | grep -oP '(?<=strong>).*(?=</strong.*)')"
- #Kernels >= 5.4 No longer builds on gcc-7.4 should be fixed by gcc-7.5
-    - compiler: gcc
-      addons:
-        apt:
-          packages:
-            - gcc-8
-      env: COMPILER=gcc-8 KVER=5.6.5
-    - compiler: gcc
-      addons:
-        apt:
-          packages:
-            - gcc-8
-      env: COMPILER=gcc-8 KVER=5.5.18
-    - compiler: gcc
-      addons:
-        apt:
-          packages:
-            - gcc-8
- #Kernels 5.4 with minor versions > 24 are failing on amd64. Not upgrade the minot version without check https://kernel.ubuntu.com/~kernel-ppa/mainline/ 
-      env: COMPILER=gcc-8 KVER=5.4.27
-    - compiler: gcc
-      env: COMPILER=gcc-7 KVER=4.19.116
-    - compiler: gcc
-      env: COMPILER=gcc-7 KVER=4.14.176
-    - compiler: gcc
-      env: COMPILER=gcc-7 KVER=4.9.219
-    - compiler: gcc
-      env: COMPILER=gcc-7 KVER=4.4.219
-    - compiler: gcc
-      env: COMPILER=gcc-7 KVER=3.16.82
+      env: CC=gcc-8 KVER_BUILD=$KERNEL_MAINLINE