Add support for a rescue (recovery) mode from kernel cmdline

This patch adds a rescue (revovery) mode to Finit.  Simply add `rescue`
to the kernel cmdline at boot and the following steps at bootstrap will
be skipped:

- Load of services/run/task/etc in /etc/finit.conf + /etc/finit.d/*.conf
- fsck of filesystems in /etc/fstab
- Remount of / to read-write
- Mount of all filesystems in /etc/fstab
- swapon
- A few plugin hooks:
  - HOOK_ROOTFS_UP
  - HOOK_MOUNT_ERROR
  - HOOK_BASEFS_UP
- `runparts DIR`
- /etc/rc.local

Instead of loading /etc/finit.conf and /etc/finit.d/*.conf Finit loads
the file /lib/finit/rescue.conf, which can be overridden by the operator
if needed.  If this file is removed (or lost), Finit falls back to start
a simple root shell, without any login.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

configure.ac

@@ -217,6 +217,9 @@ AC_EXPAND_DIR(plugin_path, "$libdir/finit/plugins")
 AC_SUBST(plugin_path)
 AC_DEFINE_UNQUOTED(PLUGIN_PATH, "$plugin_path", [Finit plugin path])
 
+AC_EXPAND_DIR(rescue_conf, "$libdir/finit/rescue.conf")
+AC_DEFINE_UNQUOTED(RESCUE_CONF, "$rescue_conf", [Finit rescue mode .conf])
+
 # Generate all files
 AC_OUTPUT