ARM: meson: clean-up platform selection

Even if multiple board are selected through Kconfig, u-boot will only
compile one. This makes sense since compiling these targets will export
global symbols, such as board_init()

The change rework amlogic Kconfig so only one board may be selected at
a time

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>