arm: am57xx: Set fastboot variables in environment

One can obtain those variables using next commands:

    $ fastboot getvar cpu
    $ fastboot getvar secure
    $ fastboot getvar board_rev
    $ fastboot getvar userdata_size

Those variables are needed for fastboot.sh script.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>