kernel_headers: kernel headers library package

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

Bug: 143567008
Change-Id: I5808a8464ed30287b75baac9a74285bff5825fe2
[rishabhb@codeaurora.org: add logic to iterate over mandatory headers
 and only generate these headers if not already present in
 usr/include/asm.]
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>
Signed-off-by: Prateek Sood <prsood@codeaurora.org>