Allow platform specific Busybox customizations

Issue #37 - This commit adds support for customizing busybox
config on a per platform basis.

We're currently supporting only config customizations similar to
how its done for the kernel.

Custom configs can be placed under: machine/<platform>/busybox/config
and the build system will pick it from there.

The customizations are optional and the build system will use
existing defaults if its not defined by a platform.