build: Implement CONFIGDIR setting

In effort to support different builds of Aimee OS using the same
scripts, without necessarily having to fork this repository, the build
system now supports a `CONFIGDIR` setting.  When this variable is set,
files defining the target environment, such as the lists of packages to
install, the kernel configuration, the Portage configuration, etc. are
found in the path it specifes.

The reference build, for the Home Assistant Yellow board, is configured
in the `yellow` directory.  To build it, run:

```sh
CONFIGDIR=yellow ./vm-build.sh
```

repos/aimee-os/sec-policy/selinux-aimee-os/files/aimee-os.if

@@ -0,0 +1,47 @@
+## <summary>Policy for Aimee OS utilities.</summary>
+
+########################################
+## <summary>
+##	Execute system-update in the aimee_sysupdate_t
+##	domain.
+## </summary>
+## <param name="domain">
+##	<summary>
+##	Domain allowed to transition.
+##	</summary>
+## </param>
+#
+interface(`aimee_os_system_update_domtrans',`
+	gen_require(`
+		type aimee_sysupdate_t, aimee_sysupdate_exec_t;
+	')
+
+	domtrans_pattern($1, aimee_sysupdate_exec_t, aimee_sysupdate_t)
+')
+
+########################################
+## <summary>
+##	Execute system-update in the aimee_sysupdate_t
+##	domain, and allow the specified role the
+##	aimee_sysupdate_t domain.
+## </summary>
+## <param name="domain">
+##	<summary>
+##	Domain allowed to transition.
+##	</summary>
+## </param>
+## <param name="role">
+##	<summary>
+##	Role allowed access.
+##	</summary>
+## </param>
+## <rolecap/>
+#
+interface(`aimee_os_run_system_update',`
+	gen_require(`
+		type aimee_sysupdate_t;
+	')
+
+	aimee_os_system_update_domtrans($1)
+	role $2 types aimee_sysupdate_t;
+')