configs: mediatek: use CONFIG_SYS_PLATFORM
When we build U-Boot for Android, we set the property "hardware" in the U-Boot environment.
This property is used by Android, some matching are done like the fstab name for example: ${hardware}.fstab
We should set property according to the current CONFIG_SYS_PLATFORM.
CONFIG_SYS_BOARD can be set to a value not expected by Android.