[PATCH v3 1/2] assemble_image: Copy U-boot environment files for flashing NAND (controlled by user button)

* Copy a user.txt and uEnv.txt if they exist in /boot in the root fs to the boot partition,
  When the user button is pressed, U-boot chooses between these 2 files, user.txt is used
  for flashing while uEnv.txt is for a regular boot

Signed-off-by: Joel A Fernandes <agnel.joel@gmail.com>