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>
diff --git a/arch/arm/mach-omap2/Kconfig b/arch/arm/mach-omap2/Kconfig
index 62c25c4..3bb1ecb 100644
--- a/arch/arm/mach-omap2/Kconfig
+++ b/arch/arm/mach-omap2/Kconfig
@@ -74,6 +74,7 @@
 
 config TI814X
 	bool "TI814X SoC"
+	select SPECIFY_CONSOLE_INDEX
 	help
 	  Support for AM335x SOC from Texas Instruments.
 	  The AM335x high performance SOC features a Cortex-A8
@@ -81,6 +82,7 @@
 
 config TI816X
 	bool "TI816X SoC"
+	select SPECIFY_CONSOLE_INDEX
 	imply NAND_OMAP_ELM
 	imply NAND_OMAP_GPMC
 	help
@@ -90,6 +92,7 @@
 
 config AM43XX
 	bool "AM43XX SoC"
+	select SPECIFY_CONSOLE_INDEX
 	imply NAND_OMAP_ELM
 	imply NAND_OMAP_GPMC
 	imply SPL_DM
@@ -111,6 +114,7 @@
 
 config AM33XX
 	bool "AM33XX SoC"
+	select SPECIFY_CONSOLE_INDEX
 	imply NAND_OMAP_ELM
 	imply NAND_OMAP_GPMC
 	imply SPL_NAND_AM33XX_BCH