bitbake.conf: add tools required by testimage to HOSTTOOLS conditionally

bitbake.conf: add tools required by testimage to HOSTTOOLS conditionally

Add tools required by testimage to HOSTTOOLS only when testimage is
inherited. These tools, as described in the comment, are only required
by the testimage task. So this change should not have negtive effect.
This would also solve build error on hosts which miss some tool such as scp.

(From OE-Core rev: 8ca61a5464743ff85b6d26886112750d6ddd13e0)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>