Convert all of CONFIG_CONS_INDEX to Kconfig

This converts the following to Kconfig:
   CONFIG_CONS_INDEX

We have existing entries for this option in a number of places, with
different guards on them.  They're also sometimes used for things not
directly inside of the serial driver.  First, introduce a new symbol to
guard the use of CONFIG_CONS_INDEX, so that in the case where we don't
need this for the serial driver, but for some other use, we can still do
it.  Next, consolidate all of these into the single entry in
drivers/serial/Kconfig.  Finally, introduce CONS_INDEX_[023456] so that
we can imply a correct value here to make the defconfig side of this
smaller.

Signed-off-by: Adam Ford <aford173@gmail.com>
[trini: Rework a lot of the logic here, such that I took authorship from
Adam, but kept his S-o-B line]
Signed-off-by: Tom Rini <trini@konsulko.com>