Fix install locations for modprobe config and udev rules (#502)

* cmake: Fix location of modprobe.d config.

* cmake: Use cmake variables for install locations.

This commit add the following two Cmake variables to
change the install location of the udev and modprobe
configuration files:
    - STLINK_UDEV_RULES_DIR
    - STLINK_MODPROBED_DIR

* doc/compiling.md: Add info about new CMAKE variables.

* debian: Use CMAKE variables for build.