go.bbclass: Use a global Go build flags

go.bbclass: Use a global Go build flags

We now use a GOBUILDFLAGS to provide a global variable to control the
build flags to be given to Go.

This change is based on the meta-golang[1] layer. Thanks to Matt
Madison <matt@madison.systems> for his work on this.

1. https://github.com/madisongh/meta-golang

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>