Example kernel headers header_lib package

This change is foundational for converting many modules that depend on kernel
headers built via the platform build system.

Bug: 143567008
Change-Id: I8c2e33aa0b28e2ec54d924b9650c302691f9c8e8
[rishabhb@codeaurora.org: add logic to iterate over mandatory headers
 and only generate these headers if not already present in
 usr/include/asm. Also export techpack uapi headers.]
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>