arm: mediatek: merge board Kconfigs into mach-mediatek

On MediaTek boards we cannot override the SYS_BOARD / SYS_CONFIG_NAME
variables from defconfig.
This is because in board/mediatek/mtXXXX/Kconfig this value was override
by default due to the if CONFIG_TARGET_MTXXXX condition.

Merge all the Kconfigs to the mach-medatek/Kconfig.

This way:
- we only define SYS_{SOC,VENDOR} once
- all board definitions are in a single place, simplifying the build logic.

Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>