Summary: Currently, boost host build doesn't actually compile boost
libraries for the host arch. If there are host tools that would want
to link against boost, it can't right now.

This diff adds support for compiling host boost library as well.
Any packages that need to depend on a host boost library can depend on
CONFIG_boost-host-build-<lib>.

Test Plan: Compiled fbthrift and verified that it works.
used openwrt master using nbg6817

Maintainer: @ClaymorePT

Signed-off-by: Amol Bhave <ambhave@fb.com>