tty: Added a CONFIG_TTY option to allow removal of TTY

The option allows you to remove TTY and compile without errors. This
saves space on systems that won't support TTY interfaces anyway.
bloat-o-meter output is below.

The bulk of this patch consists of Kconfig changes adding "depends on
TTY" to various serial devices and similar drivers that require the TTY
layer.  Ideally, these dependencies would occur on a common intermediate
symbol such as SERIO, but most drivers "select SERIO" rather than
"depends on SERIO", and "select" does not respect dependencies.

bloat-o-meter output comparing our previous minimal to new minimal by
removing TTY.  The list is filtered to not show removed entries with awk
'$3 != "-"' as the list was very long.

add/remove: 0/226 grow/shrink: 2/14 up/down: 6/-35356 (-35350)
function                                     old     new   delta
chr_dev_init                                 166     170      +4
allow_signal                                  80      82      +2
static.__warned                              143     142      -1
disallow_signal                               63      62      -1
__set_special_pids                            95      94      -1
unregister_console                           126     121      -5
start_kernel                                 546     541      -5
register_console                             593     588      -5
copy_from_user                                45      40      -5
sys_setsid                                   128     120      -8
sys_vhangup                                   32      19     -13
do_exit                                     1543    1526     -17
bitmap_zero                                   60      40     -20
arch_local_irq_save                          137     117     -20
release_task                                 674     652     -22
static.spin_unlock_irqrestore                308     260     -48

Signed-off-by: Joe Millenbach <jmillenbach@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
---
v4: Rebasing to 3.8 rc to allow longer testing as requested by Greg
    Kroah-Hartman, and applying same checks as in v3 to new code as
    requested by Jiri Slaby.
v3: Incorporated feedback from Jiri Slaby: fixed compilation issues on non
    x86/x64 platforms by finding all calls to alloc_tty_driver and
    tty_alloc_driver, then added "depends on" or "selects" TTY to config
    options that enabled compilation of those files.  Also rebased to newer
    kernel sources.
v2: Incorporated feedback from Alan Cox: used "if TTY ... endif" to wrap
    long runs of symbols that all need "depends on TTY"; grouped all the
    stubbed-out functions together in linux/tty.h.