Bump third_party/ot-br-posix/repo from `bd15f71` to `f2a7208` #342

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 46m 24s
Build on Linux (fake, gcc_release, clang, simulated)	2h 2m 15s
Build on Linux (gcc_debug)	59m 1s
Build on Linux (coverage)	14m 54s
Build on Linux (python_lib)	20m 55s
Build on Linux (python lighting-app)	6m 50s
	5h 30m 19s

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