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/am335x_baltos_defconfig b/configs/am335x_baltos_defconfig
index e12efa5..cc26882 100644
--- a/configs/am335x_baltos_defconfig
+++ b/configs/am335x_baltos_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
+CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y