[POWERPC] Create otheros.bld next to specified output file, not in $object directory

The bootwrapper script currently generates an 'otheros.bld' file in
addition to the file specified by the -o option, when asked to build a
wrapper for PS3.

It should do that in the same directory as the output, not the directory
where the wrapper objects are kept (which might potentially not be
writable when the script runs).

Arguably, the 'otheros.bld' ought to be created with the filename
specified as the -o argument. But that's a more intrusive change.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>