Set $SHELL to a sane default value, used by BusyBox

When finit calls out to external tools, like `ifup $IFACE`, that tool in
turn may use other external tools.  On systems with BusyBox our calling
`ifup $IFACE` will result in BusyBox casting a magic spell: if $SHELL is
unset it goes looking in the file /etc/passwd for the shell of $USER
which may not be set to a Bourne compatible UNIX shell, so commands like
`ip link ...` or `run-parts ...` will fail hard.

This patch adds SHELL=/bin/sh as a sane early default.

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

src/finit.c

@@ -282,9 +282,10 @@ int main(int argc, char* argv[])
 	ctx = &loop;
 
 	/*
-	 * Set the PATH early to something sane
+	 * Set PATH and SHELL early to something sane
 	 */
 	setenv("PATH", _PATH_STDPATH, 1);
+	setenv("SHELL", _PATH_BSHELL, 1);
 
 	/*
 	 * Mount base file system, kernel is assumed to run devtmpfs for /dev