urg: handle config scripts properly

urg installs two <foo>-config scripts, so the ones installed in
$(STAGING_DIR) should be fixed up, and the ones installed in
$(TARGET_DIR) should be removed. Use the <pkg>_CONFIG_SCRIPTS
infrastructure to do this.

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