baremetal-image: Fix post process command rootfs_update_timestamp

baremetal-image: Fix post process command rootfs_update_timestamp

When running:

execute_pre_post_process(d, d.getVar(ROOTFS_POSTPROCESS_COMMAND))

rootfs_update_timestamp is run, which assumes that rootfs/${sysconfdir}
is already created (usually done through the do_rootfs task on linux).

This causes the build to fail if ${sysconfdir} does not exist.

This may be overlooked if debug-tweaks is enabled since some other
commands are added, one of which creates the required path
(see postinst_enable_logging).

See [1] for more info:

[1] https://github.com/aehs29/meta-freertos/issues/4

Signed-off-by: Alejandro Enedino Hernandez Samaniego <alejandro@enedino.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>