am335x_evm: Add CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG support

We add CONFIG_ENV_VARS_UBOOT_CONFIG,
CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG and CONFIG_BOARD_LATE_INIT to set
the variables and then fdtfile and findfdt to make us of this.  It is
now possible to do 'run findfdt' to have fdtfile be set to the value of
the dtb file to load for the board we are running on.

Signed-off-by: Tom Rini <trini@ti.com>