split BUILDTAGS out of BUILDFLAGS

Signed-off-by: Michael Hudson-Doyle <michael.hudson@linaro.org>
Use a build tag rather than -X to set IAMSTATIC

Signed-off-by: Michael Hudson-Doyle <michael.hudson@linaro.org>
use code generation for GITCOMMIT/VERSION

Signed-off-by: Michael Hudson-Doyle <michael.hudson@linaro.org>
use code generation for INITSHA1 & INITPATH too

Signed-off-by: Michael Hudson-Doyle <michael.hudson@linaro.org>