ARM: overo: use same load address as beagle xm

And leave 8M for the kernel.
U-Boot: delay checking /chosen:bootargs after remapping FDT

This avoids having to copy the strings.
For now we disregard argv[] as it is not remapped before
being used in add_stage2_driver_settings() and is not used
by the linux entry point.

This makes the overo loader panic at the same place as
the beagle xm one now, even though it fails to display
anything with the default RAM size since we allocate
the framebuffer beyond 128MB...