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/axm_defconfig b/configs/axm_defconfig
index 123ae40..69bca65 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -4,11 +4,12 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g20-taurus"
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20,MACH_TYPE=2068,BOARD_AXM"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y