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/at91sam9m10g45ek_mmc_defconfig b/configs/at91sam9m10g45ek_mmc_defconfig
index 7b4a9ec..0aaa888 100644
--- a/configs/at91sam9m10g45ek_mmc_defconfig
+++ b/configs/at91sam9m10g45ek_mmc_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9M10G45,SYS_USE_MMC"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y