mediatek: introduce KERNEL_LOADADDR to Device/Default template

mediatek: introduce KERNEL_LOADADDR to Device/Default template

We need to reset KERNEL_LOADADDR if we use it on a per-device base.
Otherwise the previous value will be kept in case a device doesn't
define KERNEL_LOADADDR and relies on the default.

Move initializing KERNEL_LOADADDR to target/linux/mediatek/image/Makefile,
similar to how it's done also on the ramips target.

This fixes image size related breakage on devices which rely on the
default value of KERNEL_LOADADDR.

While at it use 0x48000000 which is more common than the previous default
0x44000000 for the filogic subtarget.

Fixed: e7c399bee6 ("filogic: add support for ASUS TUF-AX4200")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>