Convert CONFIG_UDP_CHECKSUM to Kconfig

This converts the following to Kconfig:
   CONFIG_UDP_CHECKSUM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/M5208EVBE_defconfig b/configs/M5208EVBE_defconfig
index cc307ce..e79791c 100644
--- a/configs/M5208EVBE_defconfig
+++ b/configs/M5208EVBE_defconfig
@@ -19,6 +19,7 @@
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0x2000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_UDP_CHECKSUM=y
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_FSL=y
 CONFIG_SYS_FSL_I2C_OFFSET=0x58000