Create a maintenance branch for Boost 1.54.0
coroutine: fix typo in Jamfile

From [84622], by olli
In C++11 the shared_ptr -> bool conversion is explicit. In custom time zone code, make the cast explicit. Fixes compilation failure in C++11

From [84626], by marshall.
Backported fix for BOOST_LOG_ONCE_BLOCK_FLAG macro definition.
Thread: rollback change in #8070 as this includes a regression on windows XP.

From [84946], by viboes.