Convert CONFIG_BOOTCOUNT_ENV to Kconfig

This converts the following to Kconfig:
  CONFIG_BOOTCOUNT_ENV

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/configs/thuban_defconfig b/configs/thuban_defconfig
index 197abde..eb7e285 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -56,6 +56,7 @@
 CONFIG_BOOTP_GATEWAY=y
 CONFIG_BOOTP_SUBNETMASK=y
 CONFIG_BOOTCOUNT_LIMIT=y
+CONFIG_BOOTCOUNT_ENV=y
 CONFIG_DFU_NAND=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_NAND=y