Convert CONFIG_SPL_SERIAL_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/at91sam9n12ek_spiflash_defconfig b/configs/at91sam9n12ek_spiflash_defconfig
index af475c3..8ba0658 100644
--- a/configs/at91sam9n12ek_spiflash_defconfig
+++ b/configs/at91sam9n12ek_spiflash_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9N12,SYS_USE_SPIFLASH"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y