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/board.c b/board/ti/am335x/board.c
index d81eec9..0739e60 100644
--- a/board/ti/am335x/board.c
+++ b/board/ti/am335x/board.c
@@ -383,24 +383,19 @@
 
 void set_uart_mux_conf(void)
 {
-#ifdef CONFIG_SERIAL1
+#if CONFIG_CONS_INDEX == 1
 	enable_uart0_pin_mux();
-#endif /* CONFIG_SERIAL1 */
-#ifdef CONFIG_SERIAL2
+#elif CONFIG_CONS_INDEX == 2
 	enable_uart1_pin_mux();
-#endif /* CONFIG_SERIAL2 */
-#ifdef CONFIG_SERIAL3
+#elif CONFIG_CONS_INDEX == 3
 	enable_uart2_pin_mux();
-#endif /* CONFIG_SERIAL3 */
-#ifdef CONFIG_SERIAL4
+#elif CONFIG_CONS_INDEX == 4
 	enable_uart3_pin_mux();
-#endif /* CONFIG_SERIAL4 */
-#ifdef CONFIG_SERIAL5
+#elif CONFIG_CONS_INDEX == 5
 	enable_uart4_pin_mux();
-#endif /* CONFIG_SERIAL5 */
-#ifdef CONFIG_SERIAL6
+#elif CONFIG_CONS_INDEX == 6
 	enable_uart5_pin_mux();
-#endif /* CONFIG_SERIAL6 */
+#endif
 }
 
 void set_mux_conf_regs(void)