ramips: move KERNEL_LOADADDR into Device/Default

ramips: move KERNEL_LOADADDR into Device/Default

Commit f4a79148f8cb ("ramips: add support for ipTIME AX2004M") was
reverted due to KERNEL_LOADADDR leakage, and it seems the problem can be
mitigated by moving the variable definition into Device/Default. By this,
KERNEL_LOADADDR redefined in a device recipe will not be leaked into the
subsequent device recipes anymore and thus will remain as a per-device
variable.

Ref: cd6a6e3030ff ("Revert "ramips: add support for ipTIME AX2004M"")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>