Allow to skip kernel cleanup

For some use cases, it can be useful to keep all the kernels
and not just keep the latest one. Add a parameter that allows
it, and continue cleaning up kernels by default.

Change-Id: Ia6e6c1fa18e3724c1eb89226151d81e9e748b793

elements/redhat-common/README.rst

@@ -22,3 +22,10 @@ DIB_LOCAL_IMAGE
    by diskimage-builder. It should be a full disk image, not just a filesystem
    image.
   :Example: ``DIB_LOCAL_IMAGE=rhel-guest-image-7.1-20150224.0.x86_64.qcow2``
+
+DIB_DISABLE_KERNEL_CLEANUP
+  :Required: No
+  :Default: 0
+  :Description: Specify if kernel needs to be cleaned up or not. When set to
+   true, the bits that cleanup old kernels will not be executed.
+  :Example: DIB_DISABLE_KERNEL_CLEANUP=1

elements/redhat-common/environment.d/50-redhat-common

@@ -0,0 +1 @@
+export DIB_DISABLE_KERNEL_CLEANUP=${DIB_DISABLE_KERNEL_CLEANUP:-0}

elements/redhat-common/finalise.d/01-clean-old-kernels

@@ -6,6 +6,11 @@ fi
 set -eu
 set -o pipefail
 
+if [ $DIB_DISABLE_KERNEL_CLEANUP -ne 0 ]; then
+    echo "Skipping kernel cleanup as configured"
+    exit 0
+fi
+
 YUM=${YUM:-yum}
 
 if [[ ${YUM} == "dnf" ]]; then