configs: am57xx: change default board name to beagle_x15

beagleboard x15 is the first supported platform variant of am57xx
in AOSP (android open source project) now.

changing board name to stay in consistent with aosp target name.

This patch was sent to upstream: [1].

[1] https://lists.denx.de/pipermail/u-boot/2018-August/339092.html



Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>
Change-Id: Ib8a46f0824e3c9a18b5ff733fec66174e8e6a302