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/am43xx_evm_ethboot_defconfig b/configs/am43xx_evm_ethboot_defconfig
index a1ab32c..bea1d0c 100644
--- a/configs/am43xx_evm_ethboot_defconfig
+++ b/configs/am43xx_evm_ethboot_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_AM43XX=y
 CONFIG_TARGET_AM43XX_EVM=y
+CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,NAND"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y