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