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/configs/am335x_evm_spiboot_defconfig b/configs/am335x_evm_spiboot_defconfig
index 389d1d8..a6188ea 100644
--- a/configs/am335x_evm_spiboot_defconfig
+++ b/configs/am335x_evm_spiboot_defconfig
@@ -1,4 +1,5 @@
 CONFIG_SPL=y
-CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,SPI_BOOT"
+CONFIG_SYS_EXTRA_OPTIONS="SPI_BOOT"
+CONFIG_CONS_INDEX=1
 +S:CONFIG_ARM=y
 +S:CONFIG_TARGET_AM335X_EVM=y