am335x_evm: Convert CONFIG_CONS_INDEX into a menu choice

- Drop CONFIG_SERIAL[1-6] and use CONFIG_CONS_INDEX tests instead
- Add choice and help text to board/ti/am335x/Kconfig
- Correct comment about IDK in board/ti/am335x/mux.c
- Remove am335x_evm_uart* defconfig files as they're just variations
  on a config option now.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/board/ti/am335x/Kconfig b/board/ti/am335x/Kconfig
index c3b61af..0e5149c 100644
--- a/board/ti/am335x/Kconfig
+++ b/board/ti/am335x/Kconfig
@@ -20,4 +20,13 @@
 	string
 	default "am335x_evm"
 
+config CONS_INDEX
+	int "UART used for console"
+	default 1
+	help
+	  The AM335x SoC has a total of 6 UARTs (UART0 to UART5 as referenced
+	  in documentation, etc) available to it.  Depending on your specific
+	  board you may want something other than UART0 as for example the IDK
+	  uses UART3 so enter 4 here.
+
 endif