bcm53xx: put kernel into TRX when building initramfs images

Many (most?) devices can't boot raw kernel using tftp and probably none
allows flashing it. It's way more usable to have TRX with kernel
containing initrams as such an image can be actually flashed.

An exception are Buffalo devices which have recovery mode with support
for booting kernels over TFTP. For them keep building default images.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>