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_defconfig b/configs/am43xx_evm_defconfig
index 0be9bb7..eb5b7fd 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -2,6 +2,7 @@
 CONFIG_AM43XX=y
 CONFIG_TARGET_AM43XX_EVM=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
+CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="am437x-gp-evm"
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="CONS_INDEX=1,NAND"