Convert CONFIG_SYS_FLASH_CHECKSUM to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_FLASH_CHECKSUM

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/M5249EVB_defconfig b/configs/M5249EVB_defconfig
index f99375b..7430104 100644
--- a/configs/M5249EVB_defconfig
+++ b/configs/M5249EVB_defconfig
@@ -28,4 +28,5 @@
 CONFIG_SYS_FLASH_CFI_WIDTH_16BIT=y
 CONFIG_SYS_FLASH_PROTECTION=y
 CONFIG_SYS_FLASH_CFI=y
+CONFIG_SYS_FLASH_CHECKSUM=y
 CONFIG_MCFUART=y