iproute2: support eBFP/XDP object file loading, simplify linking libelf

iproute2: support eBFP/XDP object file loading, simplify linking libelf

Add build and runtime dependencies on libelf, allowing tc and ip-full
to load BPF and XDP object files respectively.

Define package 'tc' as a singleton package variant, which can be used to
enable additional functionality limited only to tc. Also set ip-tiny
as the default 'ip' variant.

Preserve optionality of libelf by having configuration script follow the
HAVE_ELF environment variable, used similarly to the HAVE_MNL variable.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>