iproute2: simplify linking libelf for eBFP/XDP object file support

Simplify build and runtime dependencies on libelf, which allows tc and ip
to load BPF and XDP object files respectively.

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>

package/network/utils/iproute2/Makefile

@@ -49,7 +49,7 @@ $(call Package/iproute2/Default)
  VARIANT:=full
  PROVIDES:=ip
  ALTERNATIVES:=300:/sbin/ip:/usr/libexec/ip-full
- DEPENDS:=+libnl-tiny +(PACKAGE_devlink||PACKAGE_rdma):libmnl
+ DEPENDS:=+libnl-tiny +libelf +(PACKAGE_devlink||PACKAGE_rdma):libmnl
 endef
 
 define Package/tc
@@ -100,6 +100,14 @@ ifeq ($(BUILD_VARIANT),tiny)
   IP_CONFIG_TINY:=y
 endif
 
+ifeq ($(BUILD_VARIANT),full)
+  HAVE_ELF:=y
+endif
+
+ifeq ($(BUILD_VARIANT),tc)
+  HAVE_ELF:=y
+endif
+
 ifdef CONFIG_PACKAGE_devlink
   HAVE_MNL:=y
 endif
@@ -121,6 +129,7 @@ MAKE_FLAGS += \
 	KERNEL_INCLUDE="$(LINUX_DIR)/user_headers/include" \
 	SHARED_LIBS="" \
 	IP_CONFIG_TINY=$(IP_CONFIG_TINY) \
+	HAVE_ELF=$(HAVE_ELF) \
 	HAVE_MNL=$(HAVE_MNL) \
 	IPT_LIB_DIR=/usr/lib/iptables \
 	XT_LIB_DIR=/usr/lib/iptables \

package/network/utils/iproute2/patches/145-keep_libelf_optional.patch

@@ -0,0 +1,11 @@
+--- a/configure
++++ b/configure
+@@ -228,7 +228,7 @@ EOF
+
+ check_elf()
+ {
+-    if ${PKG_CONFIG} libelf --exists; then
++    if [ "${HAVE_ELF}" = "y" ] && ${PKG_CONFIG} libelf --exists; then
+ 	echo "HAVE_ELF:=y" >>$CONFIG
+ 	echo "yes"
+