erofs output (#4162)

* add erofs as output option

Signed-off-by: Avi Deitcher <avi@deitcher.net>

* unify nearly identical functions

Signed-off-by: Avi Deitcher <avi@deitcher.net>

---------

Signed-off-by: Avi Deitcher <avi@deitcher.net>

test/cases/000_build/000_formats/014_kernel+erofs/test.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# SUMMARY: Check that kernel+squashfs output format is generated
+# LABELS:
+
+set -e
+
+# Source libraries. Uncomment if needed/defined
+#. "${RT_LIB}"
+. "${RT_PROJECT_ROOT}/_lib/lib.sh"
+
+NAME=check
+
+clean_up() {
+	rm -f ${NAME}*
+}
+
+trap clean_up EXIT
+
+linuxkit build --format kernel+erofs --name "${NAME}" ../test.yml
+[ -f "${NAME}-kernel" ] || exit 1
+[ -f "${NAME}-erofs.img" ] || exit 1
+[ -f "${NAME}-cmdline" ] || exit 1
+
+exit 0