Support kargs.d directories for default kargs

Add the following config directories for setting default kernel
arguments:

/etc/ostree/kargs.d (host config)
/usr/lib/ostree-boot/kargs.d (base config)

These directories contain files whose contents consist of a karg
snippet, which is a collection kernel parameter keys and values.
Example of a snippet:

```
KEY=VALUE ANOTHERKEY SAMEKEY=VALUE2 FIRSTKEY=1 SAMEKEY=VALUE3 SECONDKEY=2 KEY= KEY.KEY-KEY_KEY="some space-separated values"
```

The bootconfig key `ostree-kargs-generated-from-config` indicates
whether the kargs were generated by ostree from the kargs.d directories
(`true`), or copied from the previous deployment (`false`). Editing the
kargs through the command line (e.g. `ostree admin deploy --karg=`) will
set the flag to `false`, and kargs will be copied from the previous
deployment for subsequent deployments.

Closes: ostreedev#479

Makefile-tests.am

@@ -98,6 +98,7 @@ _installed_or_uninstalled_test_scripts = \
 	tests/test-admin-deploy-syslinux.sh \
 	tests/test-admin-deploy-2.sh \
 	tests/test-admin-deploy-karg.sh \
+	tests/test-admin-deploy-karg-default.sh \
 	tests/test-admin-deploy-switch.sh \
 	tests/test-admin-deploy-etcmerge-cornercases.sh \
 	tests/test-admin-deploy-uboot.sh \