package/systemd: fix paths to runtime utilities

package/systemd: fix paths to runtime utilities

systemd's configure is looking in $PATH to find utilities that will be
needed at runtime.

Usually, those utilties, when installed on the host, will be found in
the same path they would be present on the target. For example,
/usr/bin/mount on the host would also be /usr/bin/mount on the target,
and all is find.

Except when we need to install a host variant of util-linux, which will
install mount in $(HOST_DIR), in which case systemd's configure would
find that one.

Of course, it is also very well possible that those utilities are not
installed on the host in the same location they would be on the target,
in case a user has manually installed some of those (e.g. in /usr/local/
or in /opt/)

Forcibly set the path to those utilities, as they are expected to be on
the target.

For kexec, we can set it even though we do not depend on it (yet).
systemd will appropriately test it at runtime.

For quota, we point to non-existing files, so as to catch errors at
runtime. It is to be noted that quotacheck is optional, while quotaon
does not seem to be (a service file is always installed, that uses it).

Note: utilties listed in the order they appear in configure.ac

Reported-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>