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 <sam@ravnborg.org>
Cc: Oskar Schirmer <os@emlix.com>
Cc: Mikael Starvik <mikael.starvik@axis.com>