package/pkg-golang: new package infrastructure

package/pkg-golang: new package infrastructure

This patch adds a new infrastructure for golang based packages.

Signed-off-by: Angelo Compagnucci <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>
Reviewed-by: "Yann E. MORIN" <[email protected]>
[Arnout:
 - Rewrap comments to 80 columns.
 - Create a global definition of GO_TARGET_ENV.
 - <PKG>_GO_ENV is appended to the default env instead of replacing it.
 - Add a note to inner-golang-package that only target is supported.
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>