samples/bpf: Add sysroot support

Basically it only enables that was added by previous couple fixes.
Sysroot contains correct libs installed and its headers. Useful when
working with NFC or virtual machine.

Usage example:

clean (on demand)
    make ARCH=arm -C samples/bpf clean
    make ARCH=arm -C tools clean
    make ARCH=arm clean

configure and install headers:

    make ARCH=arm defconfig
    make ARCH=arm headers_install

build samples/bpf:
    make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- samples/bpf/ \
    SYSROOT="path/to/sysroot"

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191011002808.28206-15-ivan.khoronzhuk@linaro.org

samples/bpf/Makefile

@@ -187,6 +187,11 @@ TPROGS_CFLAGS += -I$(srctree)/tools/lib/
 TPROGS_CFLAGS += -I$(srctree)/tools/include
 TPROGS_CFLAGS += -I$(srctree)/tools/perf
 
+ifdef SYSROOT
+TPROGS_CFLAGS += --sysroot=$(SYSROOT)
+TPROGS_LDFLAGS := -L$(SYSROOT)/usr/lib
+endif
+
 TPROGCFLAGS_bpf_load.o += -Wno-unused-variable
 
 TPROGS_LDLIBS			+= $(LIBBPF) -lelf