kbuild: introduce destination-y for exported headers

kbuild: introduce destination-y for exported headers

xtensa and arm have asked for a possibility to export headers
and locate them in a specific directory when exported.
Introduce destiantion-y to support this.

This patch in additiona adds some limited
documentation for the variables used for exported headers.

Signed-off-by: Sam Ravnborg <[email protected]>
Cc: Oskar Schirmer <[email protected]>
Cc: Mikael Starvik <[email protected]>