xenomai: install in /usr

By default, Xenomai installs everything with --prefix=/usr/xenomai,
and passing --prefix=/usr doesn't work because installing Xenomai
headers in /usr/include creates conflicts with other headers. However,
passing --prefix=/usr and --includedir=/usr/include/xenomai just works
fine. This allows to use the default configure command of the
AUTOTARGETS infrastructure, and allows to install the Xenomai headers
and libraries into more usual locations. Furthermore, it allows to
remove the documentation/headers removal hooks, as well as the
ld.so.conf hook since now everything is installed in standard
locations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>