tools, bpf: Fix build for 'make -s tools/bpf O=<dir>'

Building selftests with 'make TARGETS=bpf kselftest' was fixed in commit
55d554f5d140 ("tools: bpf: Use !building_out_of_srctree to determine
srctree"). However, by updating $(srctree) in tools/bpf/Makefile for
in-tree builds only, we leave out the case where we pass an output
directory to build BPF tools, but $(srctree) is not set. This
typically happens for:

    $ make -s tools/bpf O=/tmp/foo
    Makefile:40: /tools/build/Makefile.feature: No such file or directory

Fix it by updating $(srctree) in the Makefile not only for out-of-tree
builds, but also if $(srctree) is empty.

Detected with test_bpftool_build.sh.

Fixes: 55d554f5d140 ("tools: bpf: Use !building_out_of_srctree to determine srctree")
Signed-off-by: Quentin Monnet <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Jakub Kicinski <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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