samples/bpf: Allow building with custom CFLAGS/LDFLAGS

Currently, it is not possible to specify custom flags when building
samples/bpf. The flags are defined in TPROGS_CFLAGS/TPROGS_LDFLAGS
variables, however, when trying to override those from the make command,
compilation fails.

For example, when trying to build with PIE:

    $ make -C samples/bpf TPROGS_CFLAGS="-fpie" TPROGS_LDFLAGS="-pie"

This is because samples/bpf/Makefile updates these variables, especially
appends include paths to TPROGS_CFLAGS and these updates are overridden
by setting the variables from the make command.

This patch introduces variables TPROGS_USER_CFLAGS/TPROGS_USER_LDFLAGS
for this purpose, which can be set from the make command and their
values are propagated to TPROGS_CFLAGS/TPROGS_LDFLAGS.

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/2d81100b830a71f0e72329cc7781edaefab75f62.1698213811.git.vmalik@redhat.com

samples/bpf/Makefile

@@ -150,6 +150,9 @@ always-y += ibumad_kern.o
 always-y += hbm_out_kern.o
 always-y += hbm_edt_kern.o
 
+TPROGS_CFLAGS = $(TPROGS_USER_CFLAGS)
+TPROGS_LDFLAGS = $(TPROGS_USER_LDFLAGS)
+
 ifeq ($(ARCH), arm)
 # Strip all except -D__LINUX_ARM_ARCH__ option needed to handle linux
 # headers when arm instruction set identification is requested.
@@ -316,7 +319,7 @@ XDP_SAMPLE_CFLAGS += -Wall -O2 \
 		     -I$(LIBBPF_INCLUDE) \
 		     -I$(src)/../../tools/testing/selftests/bpf
 
-$(obj)/$(XDP_SAMPLE): TPROGS_CFLAGS = $(XDP_SAMPLE_CFLAGS)
+$(obj)/$(XDP_SAMPLE): TPROGS_CFLAGS = $(XDP_SAMPLE_CFLAGS) $(TPROGS_USER_CFLAGS)
 $(obj)/$(XDP_SAMPLE): $(src)/xdp_sample_user.h $(src)/xdp_sample_shared.h
 # Override includes for trace_helpers.o because __must_check won't be defined
 # in our include path.