platform not supported error fix and boost 1.56.0 support

Added support for boost 1.56.0.

Trying to build boost 1.56.0 thows following error:

libs/context/src/unsupported.cpp:7:2: error: #error "platform not supported"

To fix this issue Boost.Context requires extra parameters to bjam call:

abi=aapcs 
binary-format=elf
address-model=32 
architecture=arm