board: samsung: e850-96: Add bootdev var to choose boot device

Provide a way for the user to select which storage to load the LDFW
firmware from, by setting the corresponding environment variables:
  - bootdev: block device interface name
  - bootdevnum: block device number
  - bootdevpart: partition number

This might be useful when the OS is flashed and booted from a different
storage device than eMMC (e.g. USB flash drive). In this case it should
be sufficient to just set:

    => setenv bootdev usb
    => env save

assuming that the USB drive layout follows the same partitioning scheme
as defined in $partitions.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>