knxd: streamline makefile

knxd: streamline makefile

- use https URL for fetching sources from GitHub, otherwise cloning
  sources could stall buildbots by asking to accept a/the SSH host key
- do not _depend_ on DEPENDS but _select_ them, so the package(s) always
  appear in menuconfig, not only when all dependencies are already
  selected --> dependencies are automatically pulled in when package
  is selected by user
- use PKG_INSTALL
- factor out the libeibclient library as own package
- use CONFIGURE_ARGS instead of dedicated Build/Configure
- same for TARGET_CFLAGS and Build/Compile
- do not include /etc/functions, already included by /etc/rc.common

Signed-off-by: Michael Heimpold <mhei@heimpold.de>