tooling: extracts common install-definitions into a central include

tooling: extracts common install-definitions into a central include

After adding install-rules to the tools- and root-Makefile we have some
redundancies in some variables and definitions (e.g.: $(INSTALL)). To fix
this, extract all duplications and general definitions concerning the
install-process into one central Makefile-include `install.mk`.

This also unifies the way directories are created for
installation-rules. Previously we would just create them every time
without checking if that is needed. Now we add them as prerequisite to
the install-rule and generate a rule for each of them in `install.mk`.
This way `make` will figure out if the need to be created. This also
lowers the verboseness of the Makefile.

Signed-off-by: Benjamin Block <bebl@mageta.org>
[Jan: moved install.mk to scripts folder]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>