Xdp-tools modifications

Modifications made to the xdp-tools repo in order to get everything to build and install properly

modified configure file to change include directory

from
     LIBBPF_INCLUDE_DIR='$(LIB_DIR)/libbpf/src/root/usr/include'
to 
     LIBBPF_INCLUDE_DIR='$(LIB_DIR)/libbpf/src/root/include'

modified lib/Makefile to enable installation of libbpf (get all indents right!)

all: $(SUBDIRS) libxdp   ------>  all: $(SUBDIRS) libxdp libbpf
 
install: libxdp_install  ------>  install: libxdp_install libbpf_install

libbpf: $(OBJECT_LIBBPF) ------>

.PHONY: libbpf_install
libbpf_install: libbpf
       install -m 0755 -d $(DESTDIR)$(HDRDIR)
       $(MAKE) -C libbpf/src install

.PHONY: libbpf
 libbpf: $(OBJECT_LIBBPF)
       @echo; echo "  $@"; $(MAKE) -C $@/src

modified lib/libxdp/Makefile by adding these lines to the "install" target to enable installation of header files

$(Q)install -d -m 0755 $(DESTDIR)$(HDRDIR)/xdp
$(Q)install -m 0644 $(LIB_HEADERS) $(DESTDIR)$(HDRDIR)/xdp