Convert CONFIG_BOOTCOUNT_EXT to Kconfig

This converts the following to Kconfig:
  CONFIG_BOOTCOUNT_EXT

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
diff --git a/configs/ge_b450v3_defconfig b/configs/ge_b450v3_defconfig
index 2e28f89..ee1cc43 100644
--- a/configs/ge_b450v3_defconfig
+++ b/configs/ge_b450v3_defconfig
@@ -27,6 +27,8 @@
 CONFIG_DOS_PARTITION=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTCOUNT_LIMIT=y
+CONFIG_BOOTCOUNT_EXT=y
+CONFIG_SYS_BOOTCOUNT_EXT_DEVPART="1:5"
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_NETDEVICES=y