dhcp: build shared libraries

dhcp: build shared libraries

When enable bind threads support, it fails to compile dhcp:

| tmp/work/armv5e-poky-linux-gnueabi/bind/9.10.3-P3-r0/build/lib/isc/pthreads/../../../
| ../bind-9.10.3-P3/lib/isc/pthreads/thread.c:64: undefined reference to `pthread_create'

Enable build shared libraries for bind and dhcp to fix the build
failure. And the patch is ported from Fedora.

Add sub-package dhcp-libs to package shared libraries.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>