selftests/bpf: Support out-of-tree vmlinux builds for VMLINUX_BTF

Add detection of out-of-tree built vmlinux image for the purpose of
VMLINUX_BTF detection. According to Documentation/kbuild/kbuild.rst, O takes
precedence over KBUILD_OUTPUT.

Also ensure ~/path/to/build/dir also works by relying on wildcard's resolution
first, but then applying $(abspath) at the end to also handle
O=../../whatever cases.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200304184336.165766-1-andriin@fb.com

tools/testing/selftests/bpf/Makefile

@@ -129,10 +129,13 @@ $(OUTPUT)/test_stub.o: test_stub.c $(BPFOBJ)
 	$(call msg,CC,,$@)
 	$(CC) -c $(CFLAGS) -o $@ $<
 
-VMLINUX_BTF_PATHS := $(abspath ../../../../vmlinux)			\
-			       /sys/kernel/btf/vmlinux			\
-			       /boot/vmlinux-$(shell uname -r)
-VMLINUX_BTF:= $(firstword $(wildcard $(VMLINUX_BTF_PATHS)))
+VMLINUX_BTF_PATHS := $(if $(O),$(O)/vmlinux)				\
+		     $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)	\
+		     ../../../../vmlinux				\
+		     /sys/kernel/btf/vmlinux				\
+		     /boot/vmlinux-$(shell uname -r)
+VMLINUX_BTF:= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
+
 $(OUTPUT)/runqslower: $(BPFOBJ)
 	$(Q)$(MAKE) $(submake_extras) -C $(TOOLSDIR)/bpf/runqslower	\
 		    OUTPUT=$(SCRATCH_DIR)/ VMLINUX_BTF=$(VMLINUX_BTF)   \