host/etc/inittab: use -c when calling tmux

src/host/rootfs/etc/inittab

@@ -22,8 +22,8 @@ null::sysinit:/bin/ln -sf /proc/self/fd/2 /dev/stderr
 # now run any rc scripts
 ::sysinit:/etc/init.d/rcS
 
-# Put a shell on serial and console
-console::respawn:-/bin/bash
+# Put a shell on serial and tmux on console
+console::respawn:-/bin/bash -c /bin/tmux
 ttyS0::respawn:-/bin/bash
 
 # Stuff to do for the 3-finger salute