cgpt: fix spurious "None" output

Some functions don't return anything (they're commands to do something
rather than get something), so have the code swallow "None" return values.

BUG=None
TEST=`./build_library/cgpt.py debug usb disk_layout.json` no longer finishes with "None"

Change-Id: Ice95250be51241f1a73936fe3ef0544d7777a1d8
Reviewed-on: https://gerrit.chromium.org/gerrit/50080
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed