configs: meson64_android: add board specific env settings

This allows us to define extra board variables, such as "board" and
"board_name".

Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20211122152207.219023-2-mkorpershoek@baylibre.com