env: enable accessing the environment in an EXT4 partition

env: enable accessing the environment in an EXT4 partition

For example to store the environment in a file named "/uboot.env" in MMC
"0", where partition "1" contains the EXT4 filesystem, the following
configs should be added to the board's default config:

  CONFIG_ENV_IS_IN_EXT4=y
  CONFIG_ENV_EXT4_DEVICE_AND_PART="0:1"
  CONFIG_ENV_EXT4_FILE="/uboot.env"
  CONFIG_ENV_EXT4_INTERFACE="mmc"

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
[trini: Fix some line over 80 chars issues]
Signed-off-by: Tom Rini <trini@konsulko.com>