rootfs_rpm: Escape the backtick to ensure the ls runs on the target

rootfs_rpm: Escape the backtick to ensure the ls runs on the target

This change ensures that the ls /etc/rpm-postinsts runs in the target
at first boot time, rather than at the creation time of the script on
the host.

This was causing the following error in the rootfs log:

+ install -d /srv/ssd/sgw/machines/fri2/tmp/work/fri2-poky-linux/core-image-minimal/1.0-r0/rootfs//etc/rcS.d
+ i=
+ ls /etc/rpm-postinsts/
ls: cannot access /etc/rpm-postinsts/: No such file or directory

(From OE-Core rev: e893cf0b3843701e80f5c9f47be04d1a88e5ed68)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>