This commit updates Boost to version 1.71.0 and disables Boost.Context
 for arc and mips64 architectures, since either jump_fcontext or
 getcontext are undefined for those architectures.

It also fixes a bug were Boost.Fiber was not properly disabled for
mips32 and mips64 architectures.

Boost.Coroutine2 option was removed since it was redundant. By selecting
the Coroutine package, Coroutine2 is also installed.

Boost.Fiber has been disabled for target brcm47xx_generic and brcm47xx_legacy
due to misssing opcode support from instruction set.

Boost 1.71.0 brings a new header-only library
 - Boost.Variant2 [1]
   -> A never-valueless, strong guarantee implementation of
      std::variant, from Peter Dimov.

More info about Boost 1.71.0 can be found at the usual place [2].

[1]: https://www.boost.org/doc/libs/1_71_0/libs/variant2/doc/html/variant2.html
[2]: https://www.boost.org/users/history/version_1_71_0.html

Signed-off-by: Carlos Miguel Ferreira <carlosmf.pt@gmail.com>