ARM: rework amlogic configuration

Rework the board SYS_BOARD, SYS_VENDOR and SYS_CONFIG_NAME setup by moving
the board Kconfig into the mach-meson Kconfig to make it easier to add
new boards for a SoC architecture and add a custom config header or custom
board handler for a platform.

This drops the board CONFIGs and the duplicate boards configs headers in
favor of a single meson64.h config header.

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