kernel-install: Add KERNEL_INSTALL_NOOP (#4103)

Will be used by rpm-ostree (and likely lorax) to suppress
RPM->kernel->%posttrans->dracut runs, and basically everything
else this script is doing.

I'll also likely change the `kernel.spec` to respect this as well.

src/kernel-install/kernel-install

@@ -61,6 +61,13 @@ for i in "$@"; do
     fi
 done
 
+# KERNEL_INSTALL_NOOP may be used by programs like lorax and rpm-ostree which
+# want to install a kernel (indirectly via RPM), but control generation of the
+# initramfs.  In general, OSTree takes over kernel management too.
+if test -n "${KERNEL_INSTALL_NOOP:-}"; then
+    exit 0
+fi
+
 if [[ "${0##*/}" == 'installkernel' ]]; then
     COMMAND='add'
 else