boost: add option to build boost_coroutine

It was disabled by default by commit
611dfe6c because it caused some build
failures on AArch64 and SuperH. However, the coroutine module needs
the context module, which is only available on certain architectures.

We can therefore allow enabling the coroutine module, but only if the
context module is available.

[Thomas: extend commit log with more details.]

Signed-off-by: Edson Siqueira <edson.siqueira@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>