omap: Correct the fastboot product var

omap: Correct the fastboot product var

"fastboot flashall" expects "fastboot getvar product" value to be one of
values provided in android-info.txt file (in AOSP), from "require
board=" list. Before this patch, "am57xx" is returned for all AM57xx
based boards, as it's set in $board env var from SYS_BOARD in
board/ti/am57xx/Kconfig file, which is used for default implementation
of "fastboot getvar product".

In order to fix that inconsistency, let's do next:
  1. In U-Boot: override fastboot.product, reusing the value from
     $board_name
  2. In AOSP: provide values for all AM57xx boards we can use to
     device/ti/beagle_x15/board-info.txt file

This way requirements check in "fastboot flashall" will work as
expected, verifying that user tries to flash images to the board which
those images were built for.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Andrew F. Davis <afd@ti.com>