package/netopeer2: cleanup shm files after installation

package/netopeer2: cleanup shm files after installation

On install step the host tool syrepoctl is used to install some YANG
modules. Unfortunatly syrepoctl creates some files in /dev/shm folder and
does not cleanup afterwards. This files can be incompatible depending on
the used sysrepo version. This causes autobuilder failures when updating
the package [1].

To make sure we can remove this leftovers of sysrepoctl we specify a
build specific SYSREPO_SHM_PREFIX. With this the files can deleted safely
after installation is completed. This also ensures that concurrent
parallel builds will not affected mutualy.

The prfix must be unique between concurrent builds, so we use the build
directory ($(CONFIG_DIR)) to discriminate builds. It must also be unique
between top-level parallel package builds, so we also use the name of
the current package to discriminate.

Fixes:
 [1] http://autobuild.buildroot.net/results/6e559c4f98b7ed93d7b5af638264e907492a6532/

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Co-Developed-by: Yann E. MORIN <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr:
  - also use the package name as discriminant
  - expand commit log accordingly
  - rename the variable to start with the package name
  - explain why we clean up before as well
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>