apr-util: various improvements

This commit:
 * Adds licensing informations.
 * Uses the apr-1-config script instead of pointing apr-util to the
   apr build directory.
 * Ensures apu-1-config gets fixed properly using the
   <pkg>_CONFIG_SCRIPTS mechanism.
 * Ensures that libiconv gets built before apr-util if available, so
   that character set conversion features of apr-util can be enabled.

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