cmdlib.sh: factor out COSA_BUILD_WITH_BUILDAH check

Prep for next patch.

Author: jlebon

src/cmd-build

@@ -1,14 +1,15 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-if [ -n "${COSA_BUILD_WITH_BUILDAH:-}" ]; then
-    exec /usr/lib/coreos-assembler/cmd-build-with-buildah "$@"
-fi
-
 dn=$(dirname "$0")
 # shellcheck source=src/cmdlib.sh
 . "${dn}"/cmdlib.sh
 
+if should_build_with_buildah; then
+    info 'redirecting to build-with-buildah'
+    exec /usr/lib/coreos-assembler/cmd-build-with-buildah "$@"
+fi
+
 print_help() {
     cat 1>&2 <<'EOF'
 Usage: coreos-assembler build --help

src/cmd-fetch

@@ -5,6 +5,13 @@ dn=$(dirname "$0")
 # shellcheck source=src/cmdlib.sh
 . "${dn}"/cmdlib.sh
 
+if should_build_with_buildah; then
+    # shellcheck disable=SC2016
+    info 'fetch is not useful when building with buildah'
+    info 'skipping fetch'
+    exit 0
+fi
+
 FILE=cache/pkgcache-repo
 if [ -d "${FILE}" ]
 then
@@ -96,13 +103,6 @@ if [ $# -ne 0 ]; then
     fatal "ERROR: Too many arguments"
 fi
 
-if [ -n "${COSA_BUILD_WITH_BUILDAH:-}" ]; then
-    # shellcheck disable=SC2016
-    info 'fetch is not useful when using $COSA_BUILD_WITH_BUILDAH'
-    info 'Skipping fetch'
-    exit 0
-fi
-
 prepare_build
 
 lock_args=

src/cmdlib.sh

@@ -148,6 +148,17 @@ yaml2json() {
     python3 -c 'import sys, json, yaml; json.dump(yaml.safe_load(sys.stdin), sys.stdout, sort_keys=True)' < "$1" > "$2"
 }
 
+should_build_with_buildah() {
+    if [ -n "${COSA_BUILD_WITH_BUILDAH:-}" ]; then
+        if [ "${COSA_BUILD_WITH_BUILDAH:-}" = 1 ]; then
+            return 0
+        else
+            return 1
+        fi
+    fi
+    return 1
+}
+
 prepare_build() {
     preflight
     preflight_kvm