fdt: do board setup based on fdt address specified on bootm line

The last fdt patch to bootm did board setup based on the address
specified by a prior fdt address command invocation.  The bootm
code, as its call to fdt_chosen does, should use the fdt specified
by the user on the bootm command.  Note this restores full
functionality for the 8360's existing default boot environment
values, e.g. 'run nfsboot' (i.e. no having to 'fdt addr $fdtaddr'
before booting a kernel).

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>