Convert CONFIG_SPL_YMODEM_SUPPORT to Kconfig

Convert CONFIG_SPL_YMODEM_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/pengwyn_defconfig b/configs/pengwyn_defconfig
index c84cdaf..135a40f 100644
--- a/configs/pengwyn_defconfig
+++ b/configs/pengwyn_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
+CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_ETH_SUPPORT=y