configs: meson64_android: use boot part label instead of index

To boot Android, we read the gpt and assume that the partition with
index "1" will be the "boot" partition.

This might not always be the case, as there are no requirements from
Android on the partition order.

However, Android does seem to use the "boot" label quite a lot on their
public documentation [1]

Load the boot partition by label instead of by index

[1] https://source.android.com/devices/bootloader/partitions


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