env: Kconfig: Add missing dependency for ENV_IS_IN_EXT4

ENV_IS_IN_EXT4 also need to enable FS_EXT4 which is not covered in Kconfig.
Kconfig reports this as:
WARNING: unmet direct dependencies detected for EXT4_WRITE
  Depends on [n]: FS_EXT4 [=n]
  Selected by [y]:
  - ENV_IS_IN_EXT4 [=y] && !CHAIN_OF_TRUST [=n]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>