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/P1010RDB-PB_36BIT_SPIFLASH_defconfig b/configs/P1010RDB-PB_36BIT_SPIFLASH_defconfig
index 8441c63..aa63ef4 100644
--- a/configs/P1010RDB-PB_36BIT_SPIFLASH_defconfig
+++ b/configs/P1010RDB-PB_36BIT_SPIFLASH_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_DRIVERS_MISC_SUPPORT=y
 CONFIG_MPC85xx=y