serial: Rename SERIAL_SUPPORT to SERIAL

Rename these options so that CONFIG_IS_ENABLED can be used with them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/pxm2_defconfig b/configs/pxm2_defconfig
index 5938cf7..2221c1a 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -14,7 +14,7 @@
 CONFIG_SYS_MPUCLK=720
 CONFIG_TARGET_PXM2=y
 CONFIG_SPL_MMC=y
-CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_SERIAL=y
 CONFIG_BOOTCOUNT_BOOTLIMIT=3
 CONFIG_SPL=y
 CONFIG_SPL_FS_FAT=y