configs: meson64_android: use BCB for reboot to bootloader

As of today, we use a "vendor specific" secure monitor call for the
reboot reason (sm).

We should not need this. Android uses the BCB (Bootloader Control Block)
to communicate with the bootloader.

Implement "reboot into bootloader" using the standard BCB way instead of
using sm calls.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>