build-toolchain.sh: Add staging directory.

Add staging directory for things we need for building but don't want to
include in the SDK. This is where we install static libraries, some include
files etc.
build-toolchain.sh: Install ncurses to staging area

Install ncures to staging area so it doesn't end up in the SDK.
Skip building and installing things we don't need from ncurses.
build-toolchain.sh: Install ncurses to staging area

Install ncures to staging area so it doesn't end up in the SDK.
Skip building and installing things we don't need from ncurses.
Have debug and clean flags affect entire build

* build-epiphany-sdk.sh: Add two new flags; one (-d) for enabling debug
symbols and (-C) for cleaning before building. Make sure they propagate
throughout the whole build process.
* install-sdk.sh: Add clean flag (-C, --clean).