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_usbhost_boot_defconfig b/configs/am43xx_evm_usbhost_boot_defconfig
index 5bc4ef8..efe8be3 100644
--- a/configs/am43xx_evm_usbhost_boot_defconfig
+++ b/configs/am43xx_evm_usbhost_boot_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_AM43XX_EVM=y
 CONFIG_ISW_ENTRY_ADDR=0x40300350
 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="SERIAL1,CONS_INDEX=1,NAND"