package/pkg-golang: post-pone evaluation of variables

package/pkg-golang: post-pone evaluation of variables

As explained in pkg-generic.mk, all variable references inside the
inner-xxx-package should use $$(...). Otherwise, they are evaluated
too early, and will not contain the expected value. In the content of
the pkg-golang infrastructure, the <pkg>_SRC_DOMAIN, <pkg>_SRC_VENDOR
and <pkg>_SRC_SOFTWARE variables were not properly escaping their
reference to the $$($(2)_SITE) variable.

This was not visible until now, as only target Go packages were
supported, where $(2)_SITE was always defined prior to this macro
being expanded. With the upcoming support of host Go packages, we need
to fix this, as $(2)_SITE may be defined later, inherited from
$(3)_SITE.

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>
[Thomas: rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>