Fix #235: support for overriding /etc/finit.conf and /etc/finit.d

This change adds support for a new command line option finit.config=PATH
which can be used to tell Finit to start with /etc/factory.conf instead
of /etc/finit.conf.

For the complete experience a new top-level configuration file directive
`rcsd PATH` has aslo been added.  It in turn can be used by factory.conf
as follows to override /etc/finit.d:

    rcsd /etc/factory.d

Manually verified in myLinux

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

src/finit.h

@@ -122,7 +122,6 @@ extern char  *fstab;
 extern char  *sdown;
 extern char  *network;
 extern char  *hostname;
-extern char  *rcsd;
 extern char  *runparts;
 extern char  *osheading;
 extern uev_ctx_t *ctx;