Add script to build rpms and populate distro release

To better support building RPMs for multiple distribution versions, we
need a bit of help to organize the RPMs. We take the path of adding a
'rpms/7.4.1708' style directory, with the implicit knowledge this is for
CentOS and RHEL. Other distribution handling is left for the future.

To ease DOCKER_IMAGE selection for different distribution versions, the
environment variable DISTRO_VERS can be used. This simplifies a bunch of
call locations and scripting when we don't need to change the Docker
image flavor beyond this distribution version toggle.

i.e: DISTRO_VERS=el73 ./indocker.sh ./build_rpms.sh

The directory tree ends up looking like this:
rpms/7.4.1708/kmod-scoutfs-1.0-0.git.5fee207.el7.x86_64.rpm
rpms/7.3.1611/kmod-scoutfs-1.0-0.git.5fee207.el7.x86_64.rpm

kmod/.gitignore

@@ -10,5 +10,6 @@ cscope.*
 *.spec
 *.sw[po]
 rpmbuild/
+rpms/
 
 scoutfs-*.git*/

kmod/build_rpms.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -e -o pipefail
+
+# Build RPMs, populating a directory structure that indicates the OS release.
+# NOTE: This expects to run in a CentOS or RHEL environment, preferrably one of the versity
+# rpm-build Docker containers.
+
+OS_RELEASE=$(grep -oE '[0-9]+\.[0-9]+\.[0-9]+' /etc/redhat-release)
+echo "OS RELEASE: $OS_RELEASE"
+
+make rpm
+
+rpm_dist="rpms/$OS_RELEASE"
+rm -fvr "$rpm_dist"
+mkdir -p "$rpm_dist"
+
+cp -v rpmbuild/RPMS/x86_64/kmod-scoutfs*.rpm "$rpm_dist/"