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>

board/ti/am57xx/board.c

@@ -623,6 +623,7 @@ int board_late_init(void)
 			    val);
 
 	omap_die_id_serial();
+	omap_set_fastboot_vars();
 
 	am57x_idk_lcd_detect();