powerpc/ps3: Quieten boot wrapper output with run_cmd

Add a boot wrapper script function run_cmd which will run a shell command
quietly and only print the output if either V=1 or an error occurs.

Also, run the ps3 dd commands with run_cmd to clean up the build output.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>