kamailio-5.x: update build setup

kamailio-5.x: update build setup

This switches the Makefile away from calling "make" manually to using
OpenWrt default build defines. This way PKG_BUILD_PARALLEL and
PKG_INSTALL can be used.

To achieve this the flags are added to MAKE_FLAGS. The flags are also
tidied up as per example from Alpine Linux. Like this the build is
cleaner, for instance this gets rid of the wrong share directory
("//share/kamailio" instead of "/usr/share/kamailio") and things like
multi-slashes in directory names (ex. "///usr/lib/kamailio/modules").

lib_target was removed after verifying that the correct RPATH
"/usr/lib/kamailio" is still in place without it.

CFLAGS is replaced by CC_EXTRA_OPTS (same is already done for LDFLAGS).
This way upstream source can add flags like "-Wall" to OpenWrt builds.

Extra flags LOCALBASE, SYSBASE and CROSS get removed as they aren't
needed anymore with the "Alpine Linux" flags.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>