env: Properly check for BLK support

Use CONFIG_IS_ENABLED to see if CONFIG_BLK is enabled. Otherwise
SPL compilation breaks on boards which do have CONFIG_BLK enabled but
not DM_MMC for the SPL as follows:

env/mmc.c: In function ‘init_mmc_for_env’:
env/mmc.c:164:6: warning: implicit declaration of function ‘blk_get_from_parent’; did you mean ‘efi_get_ram_base’? [-Wimplicit-function-declaration]
  if (blk_get_from_parent(mmc->dev, &dev))
      ^~~~~~~~~~~~~~~~~~~
      efi_get_ram_base
env/mmc.c:164:29: error: ‘struct mmc’ has no member named ‘dev’
  if (blk_get_from_parent(mmc->dev, &dev))
                             ^~

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Simon Glass <sjg@chromium.org>