build: add build script for buildroot filesystem

Add necessary build script to support building and packaging buildroot
filesystem. Buildroot is richer in comparison to busybox and is also
easier to add user-specific binaries, libraries, packages or other
custom dev scripts etc.

Signed-off-by: Vivek Kumar Gautam <vivek.gautam@arm.com>
Change-Id: Ic4dfef2340221c0ef241ddfbb71ffe46cf186b13