blktrace: add back do_install method

The recent change to blktrace removed the do_install method,
since this recipe does not use autotools it needs to specifiy
a do_install activity.

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
rootfs_rpm.bbclass: fix the unexpected postinst error

If the /etc/rpm-postinsts/ directory was empty, the following error
would occur at system startup.

      ERROR: postinst /etc/rpm-postinsts/* failed

This patch fixes this issue.

[YOCTO #3767]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>