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_hs_evm_defconfig b/configs/am43xx_hs_evm_defconfig
index 9c01781..2a59a2b 100644
--- a/configs/am43xx_hs_evm_defconfig
+++ b/configs/am43xx_hs_evm_defconfig
@@ -4,6 +4,7 @@
 CONFIG_TARGET_AM43XX_EVM=y
 CONFIG_ISW_ENTRY_ADDR=0x40302ae0
 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"