[POWERPC] bootwrapper: no-gzip fixes

Commit a9903811 missed two uses of the
the .gz suffix in the wrapper script and didn't clean the additonal
possibly cached files.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>